Separate l3 db tests for l3 agent into their own test case

Separate l3 db tests for l3 agent into their own test case

Bug 1215871

This patch does a simple refactoring of test_l3_plugin, pushing out tests
aimed at validating the interactions of the l3 agent with the server.

These tests explicitly use TestL3NatPlugin, whereas all the
other tests use a configurable plugin, which might be specificed by a child
class. This might lead to confusion and possibly errors in unit tests for
child classes, especially those not using the l3 agent - for which running
these test is also superfluous.

Change-Id: Ia9ed320ea775fc548e6a5b711c67c9f3b0ae2f6d