Feature/add mailer service

Adds a mailer service to GSO and some config required in oss-params which I will update after this MR, including GAP deployment

docs/source/glossary.rst

@@ -58,6 +58,9 @@ Glossary of terms
     Simple Network Management Protocol: a protocol that's used for gathering data, widely used for network management
     and monitoring.
 
+  TWAMP
+    Two-way Active Measurement Protocol.
+
   UUID
     Universally Unique Identifier