MB-100: killed online config upgrade for xdcr settings

Which was never really working

Change-Id: I80baa22cb73d3a0c3d40bcd8ece6b08a20fcfb71
Reviewed-on: http://review.couchbase.org/29866
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
MB-1143: added server groups

Because removal of nodes requires removal of node from it's server
group I decided to create new compat mode: [2, 5].

Once all nodes are 2.5+ our node addition code becomes aware of server
groups. Node ejection code is always aware of it.

Change-Id: Ica7e6e0824c7e4b8decf4d3cfc81d64aaf21d2c8
Reviewed-on: http://review.couchbase.org/29867
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Merge commit 'b0b286e3b25a80dd5631d923f4e2ea0323052463'

* commit 'b0b286e3b25a80dd5631d923f4e2ea0323052463':
  MB-1143: added server groups
  MB-100: killed online config upgrade for xdcr settings

Conflicts:
	src/cluster_compat_mode.erl
	src/ns_online_config_upgrader.erl

Change-Id: I6dc106a60fd68a6d34ea295bad25730b1017d6be