Database exception causes UnboundLocalError in linuxbridge-agent

When database exception raises from update_device_down,
the linuxbridge-agent doesn't deal with them in a proper way
that causes accessing not-existed local variables.

A straightforward workaround is to set it as None by default
and verify its value then.

Change-Id: I32a9467876a619a7d0ad53ff3d5d95ff977e54f4
Closes-Bug: #1311971
Merge "Database exception causes UnboundLocalError in linuxbridge-agent"