Ensure unit tests do not let looping calls roam freely

Bug 1220871

This patch does minimal changes in neutron.plugins.nicira.common.sync
providing unit tests with a reference to the looping call object, so
that they can control its lifecycle.
Also, it perform a bit of refactoring in test_l3_agent.py in the way
mocks are created and started.

Change-Id: I73a1eb8ecdb7c6d46ff12afba549dd27095b7202
Merge "Ensure unit tests do not let looping calls roam freely"