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
Fix to not send fdb updates when no port changes

Fixes bug: 1241874
Change-Id: Ibba11db0d7972dfba819347f7cb1397e1d34f218
Imported Translations from Transifex

Change-Id: If712213da453f2263b7c5f7fec5bf9f644f5b70a
Updated from global requirements

Change-Id: Ica7315effe308bd739e90f885849fac27a3c542b
Merge "Fix to not send fdb updates when no port changes"
Merge "Updated from global requirements"
Merge "Add error log for SystemExit in dhcp-agent"