MEDIUM: server: implement 'add server' cli command

Add a new cli command 'add server'. This command is used to create a new
server at runtime attached on an existing backend. The syntax is the
following one :

$ add server <be_name>/<sv_name> [<kws>...]

This command is only available through experimental mode for the moment.

Currently, no server keywords are supported. They will be activated
individually when deemed properly functional and safe.

Another limitation is put on the backend load-balancing algorithm. The
algorithm must use consistent hashing to guarantee a minimal
reallocation of existing connections on the new server insertion.
diff --git a/doc/management.txt b/doc/management.txt
index 5f88a5d..bc7071d 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1423,6 +1423,18 @@
 
     >
 
+add server <backend>/<server> [args]*
+  Instantiate a new server attached to the backend <backend>. Only supported on
+  a CLI connection running in experimental mode (see "experimental-mode on").
+  This method is still in development and may change in the future.
+
+  The <server> name must not be already used in the backend. A special
+  restriction is put on the backend which must used a compatible load-balancing
+  algorithm with consistent hashing method. A subset of keywords from the
+  server config file statement can be used to configure the server behavior.
+  They can also be specified via an existing 'default-server' statement in the
+  backend. For the moment, no keywords are supported.
+
 add ssl crt-list <crtlist> <certificate>
 add ssl crt-list <crtlist> <payload>
   Add an certificate in a crt-list. It can also be used for directories since