MB-11292: bump max allowed maxBucketCount to 8192

It's internal setting after all and self-foot-shooting is not
forbidden for this REST API endpoint.

Change-Id: I1664f265e289639c32ad2b724f470174d9659509
Reviewed-on: http://review.couchbase.org/37766
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>