cephadm: use release/autobuild .asc instead of .gpg for repo_gpgkey
release.gpg no longer exists at download.ceph.com/keys/ (moved to
/keys/old/, last updated 2020-08-21). Only release.asc is current and
maintained. autobuild.gpg still resolves but is inconsistent with this
and could rot the same way.
repo_gpgkey() now returns the .asc URLs for both release and autobuild
keys. rpm --import and apt accept ASCII-armored keys directly, so no
other code changes are needed.
Fixes 404s seen in qa/workunits/cephadm/test_repos.sh and reported by
users running cephadm add-repo on current distros.