Add _get_resource func to get any type resource in neutron
Add _get_resource func to get any type resource in neutron

The are too many similar codes in functions for getting several
types resource in neutron, we should unify them.

Change-Id: I0b50e8b2cd29262f32db6f2b94765eb65b2bb21c

openstack/octavia,openstack/octavia,openstack/octavia