Explicit hiera lookup of interpolated values

A volunteer has been having trouble with hiera not correctly resolving
some interpolated values on their VM. Attempt to use explicit hiera
lookup rather than simple interpolation when the interpolated values are
derived from other class scopes (ie ::mediawiki using a ::mysql value).

Also replace obsolete role::mediawiki references that I missed in hhvm
templates.

Change-Id: I8543ab4d5db685adb67013efa8d6341077739691