(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.

The work needed for this to be implemented has already been
completed by @joshcooper in #3391.

After review and merge, please merge or cherry-pick up to master.