test_l3_plugin: L3AgentDbInteTestCase L3AgentDbSepTestCase fails

L3AgentDbInteTestCase and L3AgentDbSepTestCase fails when they are run
independently without other test case. something like
tox -e py27 neutron.tests.unit.test_l3_plugin
It's because necessary oslo.config options aren't properly initialized
when instantiating service plugin.
Initialize config before instantiating plugin.

Change-Id: Ic6dd28e3caf8b9e3322bf2df99e67adb138cb234
Closes-Bug: #1350252
Merge "test_l3_plugin: L3AgentDbInteTestCase L3AgentDbSepTestCase fails"