php cart: dynamic, controllable php.d seeding

Bug 1082963
https://bugzilla.redhat.com/show_bug.cgi?id=1082963

Scan the cart-provided and system `php.d` directories to determine what
php modules are available, and enable them by copying them into the
deployed cartridge's `php.d` directory.

Allow users to enable or disable php modules by setting
`OPENSHIFT_PHP_{MODULENAME}_ENABLE=true/false`