I'm using a Mac, but Vagrant thinks I'm using Linux?

devstack.log

% vagrant up     
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Importing base box 'devstack-periodic-2016-05-16'...
==> default: Matching MAC address for NAT networking...
==> default: Setting the name of the VM: devstack_default_1470673449665_9413
==> default: Clearing any previously set network interfaces...
==> default: Preparing network interfaces based on configuration...
    default: Adapter 1: nat
    default: Adapter 2: hostonly
==> default: Forwarding ports...
    default: 8000 (guest) => 8000 (host) (adapter 1)
    default: 8001 (guest) => 8001 (host) (adapter 1)
    default: 8002 (guest) => 8002 (host) (adapter 1)
    default: 8003 (guest) => 8003 (host) (adapter 1)
    default: 8031 (guest) => 8031 (host) (adapter 1)
    default: 8120 (guest) => 8120 (host) (adapter 1)
    default: 8765 (guest) => 8765 (host) (adapter 1)
    default: 9200 (guest) => 9200 (host) (adapter 1)
    default: 18080 (guest) => 18080 (host) (adapter 1)
    default: 8100 (guest) => 8100 (host) (adapter 1)
    default: 8110 (guest) => 8110 (host) (adapter 1)
    default: 9876 (guest) => 9876 (host) (adapter 1)
    default: 50070 (guest) => 50070 (host) (adapter 1)
    default: 8088 (guest) => 8088 (host) (adapter 1)
    default: 22 (guest) => 2222 (host) (adapter 1)
==> default: Running 'pre-boot' VM customizations...
==> default: Booting VM...
==> default: Waiting for machine to boot. This may take a few minutes...
    default: SSH address: 127.0.0.1:2222
    default: SSH username: vagrant
    default: SSH auth method: private key
==> default: Machine booted and ready!
Got different reports about installed GuestAdditions version:
Virtualbox on your host claims:   4.2.0
VBoxService inside the vm claims: 4.3.30
Going on, assuming VBoxService is correct...
[default] GuestAdditions versions on your host (5.1.2) and guest (4.3.30) do not match.
Got different reports about installed GuestAdditions version:
Virtualbox on your host claims:   4.2.0
VBoxService inside the vm claims: 4.3.30
Going on, assuming VBoxService is correct...
The guest's platform ("linux") is currently not supported, will try generic Linux method...
Copy iso file /Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso into the box /tmp/VBoxGuestAdditions.iso
stdin: is not a tty
mount: warning: /mnt seems to be mounted read-only.
Installing Virtualbox Guest Additions 5.1.2 - guest version is 4.3.30
stdin: is not a tty
Verifying archive integrity... All good.
Uncompressing VirtualBox 5.1.2 Guest Additions for Linux...........
VirtualBox Guest Additions installer
Removing installed version 4.3.30 of VirtualBox Guest Additions...
Copying additional installer modules ...
Installing additional modules ...
vboxadd.sh: Building Guest Additions kernel modules.
update-initramfs: Generating /boot/initrd.img-3.2.0-23-generic
vboxadd.sh: You should restart your guest to make sure the new modules are actually used.
vboxadd.sh: Starting the VirtualBox Guest Additions.

You may need to restart the the Window System (or just restart the guest system)
to enable the Guest Additions.

stdin: is not a tty
Got different reports about installed GuestAdditions version:
Virtualbox on your host claims:   4.3.30
VBoxService inside the vm claims: 5.1.2
Going on, assuming VBoxService is correct...
Got different reports about installed GuestAdditions version:
Virtualbox on your host claims:   4.3.30
VBoxService inside the vm claims: 5.1.2
Going on, assuming VBoxService is correct...
vagrant_vbguest.machine_loop_guard
==> default: Checking for guest additions in VM...
    default: The guest additions on this VM do not match the installed version of
    default: VirtualBox! In most cases this is fine, but in rare cases it can
    default: prevent things such as shared folders from working properly. If you see
    default: shared folder errors, please make sure the guest additions within the
    default: virtual machine match the version of VirtualBox you have installed on
    default: your host and reload your VM.
    default: 
    default: Guest Additions Version: 4.3.30
    default: VirtualBox Version: 5.1
==> default: [vagrant-hostsupdater] Checking for host entries
==> default: Configuring and enabling network interfaces...
Vagrant attempted to execute the capability 'configure_networks'
on the detect guest OS 'linux', but the guest doesn't
support that capability. This capability is required for your
configuration of Vagrant. Please either reconfigure Vagrant to
avoid this capability or fix the issue by creating the capability.
% sw_vers
ProductName:	Mac OS X
ProductVersion:	10.11.6
BuildVersion:	15G31