engine: Added check that provider networks unused

Added a limitation that provider cannot be removed if it has a network
that is being used by a VM/template, in order not to incapacitate that
VM/template (since without a provider, the network is unusable).

Change-Id: I9e0c0c5c1cb52109b71cac29b95b25f34977a11c
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>