Issue brctl/delif only if the bridge exists

Add a check for existence of the bridge before we try cleaning
up as we see a lot of spurious exceptions in the logs especially
when rebuild_instance is in progress and terminate_instance is
issued

Closes-Bug: #1243488

Change-Id: Ib0e2e4d9506e3a8f63201d28a3fc2c910c77d3bd
Merge "Issue brctl/delif only if the bridge exists"