MB-8126: start babysitter as hidden node

It looks like there's great deal of confusion in global facility
caused by use of same name for babysitters of all nodes.

And anyways we never intended nodes() to have babysitter in it.

Change-Id: Ib116ed791f2b1b4432b73462165ac3c96fc9eeb1
Reviewed-on: http://review.couchbase.org/26090
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>