(PUP-3268) resource/service/should_not_change_the_system should only run on agents

Use the agents object to more definitely run against the proper
machines.  This test would previously error on windows runs after the agent
role is removed from the master for windows tests.  This change fixes
this issue.
Merge pull request #3415 from er0ck/maint/master/PUP-3268-resource.service.should_not_change_the_system-should_only_run_on_agents

(PUP-3268) should_not_change_the_system should only run on agents