Implementation of multi-domain relay hosts (#922, #926)

* Add new configuration for multi-domain relay hosts (#922)
 * Creates new environment variables (replacing existing AWS_SES variables)
 * Optionally allows more advanced setups using config files
* Update relay hosts during change detection (#922)
* Add helper scripts for adding relay hosts and per-domain auth
* Allow the possibility to deliver some mail directly
* adding a domain with no destination will exclude it from the
  relayhost_map and so Postfix will attempt to deliver the mail directly
* tests for setup.sh script
* tests for relay host configuration
* these tests cover the code in `start-mailserver.sh` dealing with both
  the env vars and the configuration files

setup.sh

@@ -61,6 +61,12 @@ SUBCOMMANDS:
     $0 config dkim <keysize> (default: 2048)
     $0 config ssl
 
+  relay:
+
+    $0 relay add-domain <domain> <host> [<port>]
+    $0 relay add-auth <domain> <username> [<password>]
+    $0 relay exclude-domain <domain>
+
   debug:
 
     $0 debug fetchmail
@@ -197,6 +203,27 @@ case $1 in
     esac
     ;;
 
+  relay)
+    shift
+    case $1 in
+      add-domain)
+        shift
+        _docker_image addrelayhost $@
+        ;;
+      add-auth)
+        shift
+        _docker_image addsaslpassword $@
+        ;;
+      exclude-domain)
+        shift
+        _docker_image excluderelaydomain $@
+        ;;
+      *)
+        _usage
+        ;;
+    esac
+    ;;
+
   debug)
     shift
     case $1 in