config_opts: add two ceph-rest-api-only variables for convenience

These aren't used by the C++ code at all, but in order for
rados_conf_get to find them, they need to be listed.  They're
consumed by ceph_rest_api.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #509 from dmick/wip-rest-conf

config_opts: add two ceph-rest-api-only variables for convenience

Reviewed-by: Sage Weil <sage@inktank.com>