MINOR: server: implement delete server cli command

MINOR: server: implement delete server cli command

Implement a new CLI command 'del server'. It can be used to removed a
dynamically added server. Only servers in maintenance mode can be
removed, and without pending/active/idle connection on it.

Add a new reg-test for this feature. The scenario of the reg-test need
to first add a dynamic server. It is then deleted and a client is used
to ensure that the server is non joinable.

The management doc is updated with the new command 'del server'.