engine: Refactoring in unassign network from cluster command

Refactored some code in DetachNetworkToVdsGroupCommand to reuse code
from NetworkValidator. As part of the refactoring added to the error
message the list of VMs/templates using the network, if it was used,
simply by using the internal implementation of the validation in
NetworkValidator.

Change-Id: I127615d9144084ad093ac1d20841979be8163a12
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=746912
Signed-off-by: Lior Vernia <lvernia@redhat.com>