Set a 'php' Salt grain, indicating the name of the PHP runtime

Set the Salt grain 'php' to the name of the PHP runtime, to make it easier to
select a subset of app servers for Salt-based maintenance tasks.

For example:

    # Check the installed version of HHVM on all HHVM hosts
    $ salt -G php:hhvm cmd.run 'apt-show-versions hhvm'

Change-Id: If5860da4154b7460775f1ff1432741e394e15d8a