MB-8663 Upgrade config xdcr_something keys to {xdcr, something}.

Change-Id: Ie6c3d830e6f3a94d5b027f11b39e6d2da1ed3945
Reviewed-on: http://review.couchbase.org/28186
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>