MB-9652: never rename a node on cluster leave

In order to workaround what increasingly appears to be erlang VM
deadlock we're going to avoid renaming node even if has user-supplied
address.

Original intention of that was to allow transition from manual ->
automatic ip address. Which would be otherwise impossible. I.e. in
2.1.0 once node had manually assigned address, there was no way to
change it to automatic address management.

We preserve that logic and avoid renaming node by simply taking
current manually assigned address and saving it as automatically
assigned. Thus still switching into automatic address management mode
on cluster leave.

Change-Id: I98d9d2bb152db80ac1a5c97ebb4761e6179c7003
Reviewed-on: http://review.couchbase.org/31029
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>