Decouple helper functions from L3NatDBTestCase

In order to use the helper functions i.e _create_router in L3NatDBTestCase as
a mixin one needs to also run all the unit tests within L3NatDBTestCase.
These helper functions should be decoupled in the same way that
QuantumDbPluginV2TestCase() is done.

This patch also removes any unused variables within test_l3_plugin.py

Fixes bug 1128505

Change-Id: I2efe8547aeb31d5d9eb7eab6daeb75d9c37e8713