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'.
diff --git a/doc/management.txt b/doc/management.txt
index b859f90..cee2683 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1609,6 +1609,12 @@
   you will need to provide which line you want to delete. To display the line
   numbers, use "show ssl crt-list -n <crtlist>".
 
+del server <backend>/<server>
+  Remove a server attached to the backend <backend>. Only valid on a server
+  added at runtime. The server must be put in maintenance mode prior to its
+  deletion. The operation is cancelled if the serveur still has active
+  or idle connection or its connection queue is not empty.
+
 disable agent <backend>/<server>
   Mark the auxiliary agent check as temporarily stopped.