Use the facter value of $::path for newrelic php install

Currently, the ```path``` parameter  in ```newrelic::php::newrelic_ini``` hardcodes the path for Exec{}.  This works fine for default php installations.  However, the ```newrelic-install``` binary is smart enough to find php if it's on your path.  We use RedHat Software Collections, which depends on ```$PATH```.

This pull request solves the problem:
 - Convenient for non-standard php installations
 - Allows override if php isn't in the path by default