(maint) use el 6 master for Solaris agent testing

Prior to this commit, we were using Solaris masters to test agent
acceptance. This commit standardizes our master platform for Solaris
agent testing to match that of our Windows agent testing.
Merge pull request #3394 from rji/maint/stable/el6-masters-for-win-sol-agents

(maint) use el 6 master for Solaris agent testing