[rgw][s3] Allow colon ':' in access key

When access key contains ':', e.g. `some_info:for_user',
authorization header looks like:

"AWS some_info:for_user:request_signature"

so `auth_str.find(':')` result in auth_id = "some_info",
auth_sign = "for_user:request_signature".

auth_str.rfind(':') solves this issue.

Signed-off-by: Roman Haritonov <reclosedev@gmail.com>
Merge pull request #2447 from reclosedev/s3_colon_in_access_key

[rgw][s3] Allow colon ':' in access key

Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>