MEDIUM: cli: add check-addr command

this patch allows to set server health check address at runtime. In
order to align with `addr` command, also allow to set port optionnaly.
This led to a small refactor in order to use the same function for both
`check-addr` and `check-port` commands.
for `check-port`, we however don't permit the change anymore if checks
are not enabled on the server.

This command becomes more and more useful for people having a consul
like architecture:
- the backend server is located on a container with its own IP
- the health checks are done the consul instance located on the host
  with the host IP

Signed-off-by: William Dauchy <wdauchy@gmail.com>
diff --git a/doc/management.txt b/doc/management.txt
index b74aba7..bff770e 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1842,6 +1842,10 @@
   switch a server's state regardless of some slow health checks for example.
   Note that the change is propagated to tracking servers if any.
 
+set server <backend>/<server> check-addr <ip4 | ip6> [port <port>]
+  Change the IP address used for server health checks.
+  Optionally, change the port used for server health checks.
+
 set server <backend>/<server> check-port <port>
   Change the port used for health checking to <port>