MB-7493: allow last active server if it's unhealthy

Change-Id: I689b6ee7af8ad69639e50c2d1bcab7075001d67c
Reviewed-on: http://review.couchbase.org/26007
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>