MB-9678: require rebalance after offline upgrade

After offline upgrade (as opposed to rebalance upgrade) we have all
buckets without map_opts_hash property. Which was causing our logic to
detect server group assignment changes to fail detecting that
rebalance is required.

Simplest possible solution is to simply always require rebalance if
map_opts_hash is undefined. Right after rebalance upgrade that would
be empty rebalance which is slightly suboptimal. But given it's so
much KISS-ful I'm more than ok with that small glitch.

Change-Id: I97002ac02fd6897c95b4dee4575306a5fb80eeb9
Reviewed-on: http://review.couchbase.org/30876
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>