webadmin: Add CIDR format validation

CIDR field of a new external subnet will be validated to be in correct
format (IP/prefix - i.e. 10.0.0.0/24 is valid).

Change-Id: Id847a7970113cbb5214cf20720f578fdd3911597
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>