The main `maxctrl` function now does the cluster health checks and handles
the iteration over the host list. This solves the problem of how to
execute commands on multiple hosts while still allowing recursive calls
into the common functions.
This does not, however, solve the problem of how interconnected commands
across a set of servers are handled. One way to solve this would be to
offer a second main function, `maxctrl_cluster` which would call the
callback with a set of servers instead of a single server.
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