engine: Add validation for updating a network

Validate that network isn't being used by templates when
it is being updated so templates won't contain a network
that doesn't exist or non-VM network.

Change-Id: I8e480d908aa6e3cf55c5bccd5f84a92cb9f3d6be
Signed-off-by: Moti Asayag <masayag@redhat.com>