bugzilla: introduce BUGZILLA_API_KEY
This adds BUGZILLA_API_KEY into config, which is then used for bugzilla
authentication in each call. This replaces FAS_USER + FAS_PASSWORD (formerly
used only for proposing blockers).
This change is mandated by a recent authentication change in Bugzilla:
https://listman.redhat.com/archives/bugzilla-announce-list/2022-February/msg00000.html
This patch fixes the actual bug proposal submission process, but doesn't fix
verifying bugzilla accounts for users, yet.
Also, BUGZILLA_XMLRPC config option got dropped, because it's not needed for
python-bugzilla, we can feed it with BUGZILLA_URL.
Related: https://pagure.io/fedora-qa/blockerbugs/issue/231