remove moxi support

1. remove moxi from Mad-Hatter nodes

2. make an attempt to prevent MadHatter nodes from entering cluster with enabled moxi

2.1. Per-bucket moxi might be run on pre MadHatter cluster nodes if any of the buckets has moxi_port specified. So we decided not to allow rebalancing-in MadHatter node until user manually unsets moxi_port for every such bucket. Removing moxi_port was addressed by this ticket: MB-29124

2.2. Default moxi is run on clusters from 5.0 to 6.0 when there's a 'default' bucket and passwordless 'default' user. So we might want disallow rebalancing-in MadHatter node until passwordless user is either deleted or the password is changed.

2.3. Default moxi is always run on kv nodes of pre 5.0 clusters. In this case, I guess, we just need to release-note the fact that 4.6 cluster will not have default moxi after upgrade to MadHatter.

2.4. We won't try to prevent MadHatter node to enter cluster with enabled moxi via offline upgrade, since in this case the service is already interrupted.