rgw: add civetweb as a default frontend

Fixes: #9013
Originally the configurable was empty, now setting explicitly both
fastcgi and civetweb (on port 7480).

Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge branch 'wip-9013' into giant