More SMTP customization (#1372)

* Allow SMTP auth method customization

* Add SMTP openssl_verify_mode option support

Allows one use self-signed certs with their SMTP server.

* Add SMTP enable_starttls_auto option support

scalingo.json

@@ -71,6 +71,18 @@
       "description": "Address to send emails from",
       "required": false
     },
+    "SMTP_AUTH_METHOD": {
+      "description": "Authentication method to use with SMTP server. Default is 'plain'.",
+      "required": false
+    },
+    "SMTP_OPENSSL_VERIFY_MODE": {
+      "description": "SMTP server certificate verification mode. Defaults is 'peer'.",
+      "required": false
+    },
+    "SMTP_ENABLE_STARTTLS_AUTO": {
+      "description": "Enable STARTTLS if SMTP server supports it? Default is true.",
+      "required": false
+    },
     "BUILDPACK_URL": {
       "description": "Internal scalingo configuration",
       "required": true,