Add error log for SystemExit in l3-agent

The SystemExit exception should be logged in l3-agent, the
operator may miss the useful exception messages to debug.

    * Add error log for SystemExit in l3-agent.
    * Add unit test for None and Wrong interface_driver.

Closes-Bug #1229105

Change-Id: I9ac60a528c1e88827078d552a30c8d571066d21c
Merge "Add error log for SystemExit in l3-agent"