L3 port delete prevention: do not raise if no IP on port

Bug #1104337

This patch alters the prevent_l3_port_deletion logic.
PortInUse is raised only if the port has a device owner in
(router_interface, router_gateway, floatingip) and an IP
address.
If no IP address is found on the port the port itself can
be considered stale and therefore deleted as any association
with l3 entities would be void.

Change-Id: I9d711b2a9d2cbcc8be481f1751f51fe5b9f01327