core: add locking groups for resources in export domain

Until now we used the TEMPLATE & VM locking groups to lock those
resources in data domains as well as in export domain. it lead to
unnecessary restrictions, for example: we couldn't remove template
from data domain while it is being removed from export domain.

Now that we don't have to put locks in the DB anymore, we can split the
locks of resources in data domains from the locks of those resources
in the export domain by creating two different locking groups.

This patch introduces two new locking groups: REMOTE_TEMPLATE & REMOTE_VM
which will be used to lock templates/VMs in the export domain.

This patch also removes unnecessary code from getSharedLocks method in
AddVmFromTemplateCommand class that was lately added.

Change-Id: I1877ecce86198f8c9345f5d35820121866c3dfe0
Bug-Url: https://bugzilla.redhat.com/920150
Signed-off-by: Arik Hadas <ahadas@redhat.com>