lib/repo: Change resolve_keyring_for_collection() to return a remote

lib/repo: Change resolve_keyring_for_collection() to return a remote

Instead of returning just the keyring filename, return the entire
OstreeRemote, which has the keyring filename as one of its members. This
will simplify some upcoming changes, and allows slightly improved debug
logging.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1202
Approved by: cgwalters