engine: Add validations for removing a network

The patch adds several validations for removing a network:
1. Prevent removing a management network
2. Prevent removing a network used by VMs
3. Prevent removing a network which is attached to hosts
4. Prevent removing a network which is used by templates.

Change-Id: Ia3b797f79aac104ff73d5ad21a87ede259276b4c
Signed-off-by: Moti Asayag <masayag@redhat.com>