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.
Ansible has `apt_key` and `apt_repository` modules. We use them here: https://github.com/roots/bedrock-ansible/blob/master/roles/mariadb/tasks/main.yml

These two first tasks should be able to use those I assume.