Use "objects_exist" in PortForwardingPlugin
Use "objects_exist" in PortForwardingPlugin

This is a small optimization in
"PortForwardingPlugin._check_floatingip_request". To check the existence
of any OVO, "objects_exist" is the fastest method.

Closes-Bug: #1942358
Change-Id: I5097dbdda5a17d15152c5a6c1a56f5b5037d1f70

mahak/neutron,mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,openstack/neutron