The SystemExit exception should be logged in dhcp-agent, the
operator may miss the useful exception messages to debug.
* Add error log for SystemExit in dhcp-agent.
* Add addCleanup(mock.patch.stopall) in setup()
* Remove teardown()
* Add unit test for None and nonexistent interface_driver.
Closes-Bug #1239030
Change-Id: I01bef69adecaaf8d8a919034d33c60a43e4469a7