Playbook with everything.

common_task.yml

---
- name: update cache
  apt: update_cache=yes

- name: install vim
  apt: pkg=vim-nox state=installed

file layout

$ tree
.
├── LICENSE
├── README.md
├── Vagrantfile
├── Vagrantfile.works
├── group_vars
│   └── all
├── hosts
├── roles
│   ├── common
│   │   ├── files
│   │   │   └── iptables-save
│   │   ├── handlers
│   │   │   └── main.yml
│   │   └── tasks
│   │       └── main.yml
│   ├── magento
│   │   └── tasks
│   │       └── main.yml
│   ├── mysql
│   │   ├── handlers
│   │   │   └── main.yml
│   │   ├── tasks
│   │   │   └── main.yml
│   │   └── templates
│   │       └── my.cnf.j2
│   ├── nginx
│   │   ├── handlers
│   │   │   └── main.yml
│   │   ├── tasks
│   │   │   └── main.yml
│   │   └── templates
│   │       └── default.conf
│   └── php-fpm
│       ├── handlers
│       │   └── main.yml
│       ├── tasks
│       │   └── main.yml
│       └── templates
│           ├── magento.conf
│           └── mcrypt.ini
├── site.yml

group_vars_all

$ cat group_vars/all
---
# Which version of Magento to deploy
mag_version: 1.7.0.2

# These are the Magento database settings
mag_db_name: magento
mag_db_user: magento
mag_db_password: Sento9767$

# You shouldn't need to change this.
mysql_port: 3306

# This is used for the nginx server configuration, but # access to the
# Magento site is not restricted by a # named host.
server_hostname: localhost

magento_tasks_yml

---
- name: Download Magento Downloader
  get_url: url=http://www.magentocommerce.com/downloads/assets/{{ mag_version }}/magento-{{ mag_version }}.tar.gz dest=/srv/magento-{{ mag_version }}.tar.gz

- name: Extract Archive
  command: chdir=/srv/ /bin/tar xvf magento-{{ mag_version }}.tar.gz creates=/srv/magento

- name: Change ownership of Magento installation
  file: path=/srv/magento/ owner=www-data group=www-data state=directory recurse=yes

mysql_tasks.yml

---
- name: Install Mysql package
  apt: name={{ item }} state=present
  with_items:
   - mysql-server
   - python-mysqldb

- name: Create Mysql configuration file
  template: src=my.cnf.j2 dest=/etc/my.cnf
  notify:
  - restart mysql

- name: Start Mysql Service
  service: name=mysql state=started enabled=true

- name: Create Magento database
  mysql_db: name={{ mag_db_name }} state=present

- name: Create Magento database user
  mysql_user: name={{ mag_db_user }} password={{ mag_db_password }} priv={{ mag_db_name }}.*:ALL host='localhost' state=present

- debug: msg="this is ansibleweb1_ip value {{ hostvars[ansibleweb1]['ansible_' + iface].ipv4.address }}

nginx_tasks.yml

---
- name: Install nginx
  apt: name=nginx state=present

- name: Copy nginx configuration for Magento
  template: src=default.conf dest=/etc/nginx/conf.d/default.conf
  notify: restart nginx

- name: Move default nginx configuration file
  command: mv /etc/nginx/sites-available/default /etc/nginx/sites-available/default.old creates=/etc/nginx/sites-available/default.old
  notify: restart nginx

php-fpm_tasks.yml

---
- name: Install php-fpm and deps
  apt: name={{ item }} state=present
  with_items:
    - php5
    - php5-fpm
    - php5-enchant
    - php5-idn
    - php5-mysqlnd
    - libphp-phpmailer
    - libphp-simplepie
    - php5-xmlrpc
    - php5-mcrypt
    - php5-gd
    - php5-cli

- name: Disable default pool
  command: mv /etc/php5/fpm/pool.d/www.conf /etc/php5/fpm/pool.d/www.disabled creates=/etc/php5/fpm/pool.d/www.disabled

- name: Copy php-fpm configuration
  template: src=magento.conf dest=/etc/php5/fpm/pool.d/

- name: Copy mcrypt.ini to correct location
  template: src=mcrypt.ini dest=/etc/php5/mods-available/

- name: Fix php-fpm for Ubuntu 13.10
  command: php5enmod mcrypt
  notify: restart php-fpm

site.yml

cat site.yml
---
- name: Install Magento, Nginx, and PHP-FPM on web servers
  hosts: web
  user: root
 # sudo: yes

  roles:
    - common
    - nginx
    - php-fpm
    - magento

- name: Install MySQL on DB servers
  hosts: db
  user: root

  roles:
    - mysql

As long as you're running the whole site playbook, and it's hitting the webserver first, the webserver's facts should be available in the "hosts: db" play. The only thing I see wrong is that this:

• debug: msg="this is ansibleweb1_ip value {{ hostvars[ansibleweb1]['ansible_' + iface].ipv4.address }}

Should be:

• debug: msg="this is ansibleweb1_ip value {{ hostvars['ansibleweb1']['ansible_' + iface].ipv4.address }}"