(maint) Use EL 7 masters for Solaris agent testing

For Windows acceptance testing, we've standardized on using EL 7 masters for testing, since we don't support Puppet masters running on Windows. However, we never standardized on a master platform for Solaris agent testing. This has left us with Solaris 11 agent testing using a Solaris 11 master, and Solaris 10 agent testing using an Ubuntu Trusty master.

This PR standardizes our Solaris agent testing to always use EL 7 masters, like we do elsewhere.