nixos/modules/security/acme.nix: add server option

Add a new option permitting to point certbot to an ACME Directory
Resource URI other than Let's Encrypt production/staging one.

In the meantime, we are deprecating the now useless Let's Encrypt
production flag.

nixos/modules/security/acme.nix

@@ -20,6 +20,16 @@ let
         '';
       };
 
+      server = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = ''
+          ACME Directory Resource URI. Defaults to let's encrypt
+          production endpoint,
+          https://acme-v02.api.letsencrypt.org/directory, if unset.
+        '';
+      };
+
       domain = mkOption {
         type = types.str;
         default = name;
@@ -109,7 +119,15 @@ in
 {
 
   ###### interface
-
+  imports = [
+    (mkRemovedOptionModule [ "security" "acme" "production" ] ''
+      Use security.acme.server to define your staging ACME server URL instead.
+
+      To use the let's encrypt staging server, use security.acme.server =
+      "https://acme-staging-v02.api.letsencrypt.org/directory".
+    ''
+    )
+  ];
   options = {
     security.acme = {
 
@@ -129,6 +147,16 @@ in
         '';
       };
 
+      server = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = ''
+          ACME Directory Resource URI. Defaults to let's encrypt
+          production endpoint,
+          <literal>https://acme-v02.api.letsencrypt.org/directory</literal>, if unset.
+        '';
+      };
+
       preliminarySelfsigned = mkOption {
         type = types.bool;
         default = true;
@@ -142,20 +170,6 @@ in
         '';
       };
 
-      production = mkOption {
-        type = types.bool;
-        default = true;
-        description = ''
-          If set to true, use Let's Encrypt's production environment
-          instead of the staging environment. The main benefit of the
-          staging environment is to get much higher rate limits.
-
-          See
-          <literal>https://letsencrypt.org/docs/staging-environment</literal>
-          for more detail.
-        '';
-      };
-
       certs = mkOption {
         default = { };
         type = with types; attrsOf (submodule certOpts);
@@ -198,7 +212,7 @@ in
                           ++ optionals (data.email != null) [ "--email" data.email ]
                           ++ concatMap (p: [ "-f" p ]) data.plugins
                           ++ concatLists (mapAttrsToList (name: root: [ "-d" (if root == null then name else "${name}:${root}")]) data.extraDomains)
-                          ++ optionals (!cfg.production) ["--server" "https://acme-staging-v02.api.letsencrypt.org/directory"];
+                          ++ optionals (cfg.server != null || data.server != null) ["--server" (if data.server == null then cfg.server else data.server)];
                 acmeService = {
                   description = "Renew ACME Certificate for ${cert}";
                   after = [ "network.target" "network-online.target" ];