MEDIUM: server: allow to remove servers at runtime except non purgeable

Relax the condition on "delete server" CLI handler to be able to remove
all servers, even non dynamic, except if they are flagged as non
purgeable.

This change is necessary to extend the use cases for dynamic servers
with reload. It's expected that each dynamic server created via the CLI
is manually commited in the haproxy configuration by the user. Dynamic
servers will be present on reload only if they are present in the
configuration file. This means that non-dynamic servers must be allowed
to be removable at runtime.

The dynamic servers removal reg-test has been updated and renamed to
reflect its purpose. A new test is present to check that non-purgeable
servers cannot be removed.
diff --git a/doc/management.txt b/doc/management.txt
index a6baf8e..58d06e4 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1470,6 +1470,11 @@
   init-addr method. This means that no resolution will be undertaken if a FQDN
   is specified as an address, even if the server creation will be validated.
 
+  To support the reload operations, it is expected that the server created via
+  the CLI is also manually inserted in the relevant haproxy configuration file.
+  A dynamic server not present in the configuration won't be restored after a
+  reload operation.
+
   A dynamic server may use the "track" keyword to follow the check status of
   another server from the configuration. However, it is not possible to track
   another dynamic server. This is to ensure that the tracking chain is kept
@@ -1786,10 +1791,11 @@
   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.
+  Remove a server attached to the backend <backend>. All servers are eligible,
+  except servers which are referenced by other configuration elements. 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.