xfail test_ensure_no_updates_avail to prevent dev confusion during deb build

[redshiftzero]

Description

It's confusing for people that rarely build deb packages (make build-debs) to see test_ensure_no_updates_avail fail due to the container in which we build debian packages needing updates. I think we should conditionally xfail based on the current tag - if we're on a branch or a tag that does not contain rc, then we should let the test fail. See #4533 which contains some additional relevant background.

cc @emkll on this one

[emkll]

I think this is a good idea, if ever we adopt the automatic apt upgrade path we could easily revert the change.

[sssoleileraaa]

I also see these failures during the make staging step (which are safe to ignore but alarming):

    TASK [common : Ensure the system clock is set accurately.] *********************
    fatal: [mon-staging]: FAILED! => {"changed": false, "cmd": ["ntpdate", "ntp.ubuntu.com"], "delta": "0:00:00.277859", "end": "2019-11-14 23:41:40.639235", "msg": "non-zero return code", "rc": 1, "start": "2019-11-14 23:41:40.361376", "stderr": "14 Nov 23:41:40 ntpdate[2138]: the NTP socket is in use, exiting", "stderr_lines": ["14 Nov 23:41:40 ntpdate[2138]: the NTP socket is in use, exiting"], "stdout": "", "stdout_lines": []}
    ...ignoring
    fatal: [app-staging]: FAILED! => {"changed": false, "cmd": ["ntpdate", "ntp.ubuntu.com"], "delta": "0:00:00.296617", "end": "2019-11-14 23:41:40.631107", "msg": "non-zero return code", "rc": 1, "start": "2019-11-14 23:41:40.334490", "stderr": "14 Nov 23:41:40 ntpdate[2100]: the NTP socket is in use, exiting", "stderr_lines": ["14 Nov 23:41:40 ntpdate[2100]: the NTP socket is in use, exiting"], "stdout": "", "stdout_lines": []}
    ...ignoring