Add common base class for agent functional tests

Refactor the functional tests a bit to add a common base class for agent
tests. This is needed an upcoming commit which adds a functional test
for VXLAN version checking.

Closes-Bug: #1301363

Change-Id: I438e401cbfb40e5557e5125f2a409bac5c1fd1c2
Merge "Add common base class for agent functional tests"