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 page 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.