(maint) Don't test agent on the master for Win/Sol

Windows and Solaris Beaker configs currently test the 'agent' role on
the master, typically an EL 6 or EL 7 machine. This leads to
double-testing, since we already test these platforms elsewhere.

This commit removes the 'agent' role from the master for Windows and
Solaris Beaker configs to speed up testing and reduce false positives
for these agent platforms.
Merge pull request #3413 from rji/maint/stable/remove-agent-role-from-master-win-sol

(maint) Don't test agent on the master for Win/Sol