rgw: fix manager selection when APIs customized

rgw: fix manager selection when APIs customized

When modifying rgw_enable_apis per RGW instance, such as for staticsites, you
can end up with RESTManager instance being null in some cases, which returns a
HTTP 405 MethodNotAllowed to all requests.

Example configuration to trigger the bug:
rgw_enable_apis = s3website

Backport: jewel
X-Note: Patch from Yehuda in private IRC discussion, 2016/05/20.
Fixes: http://tracker.ceph.com/issues/15973
Fixes: http://tracker.ceph.com/issues/15974
Signed-off-by: Robin H. Johnson <robin.johnson@dreamhost.com>
(cherry picked from commit 7c7a465b55f7100eab0f140bf54f9420abd1c776)