rgw: Fallback to Host header for bucket name.

rgw: Fallback to Host header for bucket name.

RGW should fallback to using the Host header as the bucket name if valid &
possible even when it is NOT a suffix match against the DNS names, or a match
against the CNAME rule.

This mirrors AWS S3 behavior for these cases (The AWS S3 does not do any DNS
lookups regardless).

Backport: jewel
Fixes: http://tracker.ceph.com/issues/15975
Signed-off-by: Robin H. Johnson <robin.johnson@dreamhost.com>