Introduce a new policy: ha-promote-on-failure (always by default)

This new policy controls if unsynchronised slaves should be promoted
after master crash. If set to `when-synced`, unsynchronised slaves
will not be promoted, keeping the state of the queue, but making it
unavailable until master node returns.

This change is supposed to make the cluster shutdown safier,
because queues can fail or be killed on shutdown.

The queues without master will be available from the management UI
and can be deleted and redeclared, but will not automatically loose
messages.

Trying to declare or passively declare the queue will result in a
timeout error. Same way as if the master was gracefully stopped with
ha-promote-on-shutdown: when-synced

[#156811690]

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