[2.1] MAAS rack (and region) should not depend on avahi-utils

[LaMont Jones]

maas-rack-controller Recommends: avahi-utils, but assumes it's there, which can result in a traceback because:
2016-09-01 22:41:16 [-] FileNotFoundError: [Errno 2] No such file or directory: '/usr/bin/avahi-browse'

If avahi-utils is not installed (and therefore avahi-browse is not present), we should be more graceful about it and not generate a traceback.

A similar thing applies to tcpdump.

[Mike Pontillo (mpontillo)]

We need to change this back to a Recommends (it is currently a Depends, because we're being too honest) and allow MAAS to fail gracefully when avahi isn't installed. I don't think it's proper that we're essentially forcing the avahi service to be installed on any MAAS region or rack controller. (Yes, it improves the MAAS user experience, but customers may have the requirement that avahi not be installed, such as for security reasons.)

[Andres Rodriguez (andreserl)]

** This is an automated message **

We believe this is no longer an issue on the latest MAAS releases. As such, we are marking this bug report as invalid. If you believe this is still an issue, please re-open the bug report and provide any relevant information.

Thanks.