core: Don't store Neutron physical network on import

This allowed Neutron physical network entries to sneak into our
DB. Except for eliminating that possibility, such existing entries are
cleared from the DB.

Change-Id: Icb8751f06690ffb34cc72f1f2e5e14e66b4d40f1
Bug-Url: https://bugzilla.redhat.com/1127687
Signed-off-by: Lior Vernia <lvernia@redhat.com>