HHVM provisioning and toggling between php-fpm

* Renamed `php-55` to `php-fpm` as it made sense ( to me ) to remove the
version number from the role as it may change in the future, and
php-fpm is a more accurate representation of what we’re doing (
especially considering we don’t have version numbers in any of the
other roles ).

* Added base php role. This will load proper php implementation in
`meta/main.yml` based upon the variable `php_implementation` located in
`vars/common.yml`. The tasks in the php role are executed after said role.

* Added `nginx-php-fpm.conf` and `nginx-hhvm.conf` to allow decoupling of
how nginx passes the request back to php. I believe the hhvm
implementation can be used for both, though we’d have to change from a
socket connection to a tcp connection. Should we choose, hhvm can be
used over a unix socket and we can standardize the file name structure
as well so as to not have to keep changing the nginx php location block.
No quotes needed around the actual `hhvm` value here.