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`

squashed commit: php_module_scan: unbound variable dependency

Obtain PHP version from ident variable `OPENSHIFT_PHP_IDENT`

`php_module_scan` function `scan_php_modules` depended on
`OPENSHIFT_PHP_VERSION` being set, but this isn't necessarily going to
be the case.

Amended `cartridge-lifecycle-php.feature` tests to include checks for
enable/disable PHP module via user env var

Added needed step to `cartridge-php_steps.rb`