core: custom fencing settings are not save to DB

3.5 version introduced a new way to define custom fencing agents.
However, there is a bug that caused custom PM agent settings not to be
saved in DB.
The UpdateVdsCommand canDoAction checks for validity of the agent and it
comapres the selected agent against all of agents supported natively by
the agent.
The code was changed to take into account custom PM agents as well in
the canDoAction validation

Change-Id: If41fb226cbb3a80aa2a524e22756a91553dec1b1
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1122473
Signed-off-by: Eli Mesika <emesika@redhat.com>