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 appending the `.ini`
contents into the file `openshift.ini`, located in deployed cartridge's
`php.d` directory.

Allow users to enable or disable php modules by setting
`OPENSHIFT_PHP_{MODULENAME}_ENABLE=true/false`. Modules are **disabled
by default**, so modules installed after an app is deployed will have to
be enabled by setting the appropriate environment variable via `rhc env-set`.

Additional changes/squashed commits

* Obtain PHP version from ident variable `OPENSHIFT_PHP_IDENT`

* `php_module_scan` function `scan_for_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`