Add config knob for cgitb traceback logs

Adds "trace_file" a configuration knob to python daemons
that defaults to previouly hardcoded value

Exceptions while accessing this log file are now cathed
in vnc_openstack, in order to avoid side effects that are
described in attached bug.

Api_server extension loading is also changed to provide
sandesh logger to vnc_openstack. vnc_openstack exceptions
messages will be logged with api-server logs (for now,
they were only printed to local trace_file and/or stdout).

Partial-Bug: #1335062

Change-Id: Id2d28e5e2ef65c93c87b080b53e838ce65c0416d
Merge "Add config knob for cgitb traceback logs"