Fix KeyError: 'L3_ROUTER_NAT' in l3 scheduler functional test

Fix KeyError: 'L3_ROUTER_NAT' in l3 scheduler functional test

The issue was introduced by commit with change ID:
Ic4df299bac83c80abf0890a81d2dd36d15993b33

In Neutron-speak we define the 'core plugin' as something like ML2, while
service plugins examples are L3, VPNaaS, etc. The code was initializing
the L3 service plugin as the core plugin, which is unexpected behavior.
The code will now use the DB core plugin base class as the core plugin,
and not initialize the service plugins. The tests will manually and locally
instantiate a L3 service plugin instance and use it.

Change-Id: I2b7d2f6ccd5fe18e322e70b4e376db23d76b9092
Closes-Bug: #1478273