tests: Improve assertNetworkExists and assertNetworkDoesntExist

Both should also verify that bridges actually exist or absent.
Currently we only assert that bridged networks are stored in libvirt.

Change-Id: Id7fd3c41634958c8e463fdce8a6498aa1a5f0ab1
Signed-off-by: ibarkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35872
Reviewed-by: Dan Kenigsberg <danken@redhat.com>