mgr/dashboard: Do not default to 'admin' as Admin Resource

mgr/dashboard: Do not default to 'admin' as Admin Resource

The current if-statement would always default to 'admin' as the admin
resource regardless of what 'mgr/dashboard/RGW_API_ADMIN_RESOURCE' is set
to.

In some cases users change this setting in the RGW for various reasons.

The dashboard currently does not support this and will cause the RGW management
not to work.

Fixes: http://tracker.ceph.com/issues/39338
Signed-off-by: Wido den Hollander <wido@42on.com>
(cherry picked from commit 83b10bac4e382a510e5fc4066b90f67bbe526844)