engine: Ability to assign multiple VMs from a pool to single user

Added a new field VmPool.maxAssignedVmsPerUser representing maximumum
number of VMs a single user can attach to from the same pool. Default value of
1 is equivalent to current behavior.

This patch also includes necessary minimal frontend changes in userportal
due to the logic of showing/hiding VmPools in userportal.

Change-Id: I3a1cc836db48d7094c2e7a03ee3a60b0b34a52a9
Signed-off-by: Martin Betak <mbetak@redhat.com>