Remove dupl. for get_resources in adv. services

There is duplication of logic related to creating API resources
for advanced services extensions. Extracted the common logic
out into a new module that can be used by the services.
There are already UT cases in the services that exercises
the logic, so no new tests were added.

Change-Id: Ib52e2de6f215a1755f11a382dc4451a05ce3b147
Closes-Bug: 1258656
Add test to port_security to test with security_groups

This patch adds a missing testcase to the port_security tests to test
for creating a port with port_security_enabled=False and passing in
a security group.

Closes-bug: #1267246
Change-Id: Ifb5a5571f016a5d7c5b5075c97dc27279cd79bb7
Merge "Remove dupl. for get_resources in adv. services"
Merge "Add test to port_security to test with security_groups"