Fix for OPENAM-2400 - review: CR-1857
AgentsRepo now makes sure that the internal cache is dirtied for agents if
their agentgroup has been updated.

git-svn-id: https://svn.forgerock.org/openam/trunk/openam@5582 0f4defcf-c51a-4c67-9f44-6fb5eba73c5d