env_keep.pp

define sudo::defaults::env_keep {
  augeas { "set env_keep defaults for ${name}":
    context => '/files/etc/sudoers',
    changes => [
      "defnode s Defaults[env_keep/var='${name}']/env_keep[var='${name}']/append 'blah'",
      'clear $s',
      "set \$s/../var[.='${name}'] '${name}'",
    ],
  }
}

sudo::defaults::env_keep { 'https_proxy': }

gistfile1.txt

augtool> print /files/etc/sudoers/
/files/etc/sudoers
/files/etc/sudoers/Defaults[1]
/files/etc/sudoers/Defaults[1]/env_keep
/files/etc/sudoers/Defaults[1]/env_keep/append
/files/etc/sudoers/Defaults[1]/env_keep/var[1] = "http_proxy"
/files/etc/sudoers/Defaults[1]/env_keep/var[2] = "editor"
/files/etc/sudoers/Defaults[1]/env_keep/var[3] = "test"
/files/etc/sudoers/Defaults[2]
/files/etc/sudoers/Defaults[2]/env_keep
/files/etc/sudoers/Defaults[2]/env_keep/append
/files/etc/sudoers/Defaults[2]/env_keep/var = "ftp_proxy"
/files/etc/sudoers/Defaults[3]
/files/etc/sudoers/Defaults[3]/env_keep
/files/etc/sudoers/Defaults[3]/env_keep/append
/files/etc/sudoers/Defaults[3]/env_keep/var = "extra"