Add error log for SystemExit in dhcp-agent

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
Merge "Add error log for SystemExit in dhcp-agent"