Makes the mongo authentication mechanism configurable from the environment

* Defaults to ':scram', the authentication mechanism supported by mongo >3.0,
  which is also supported by the devstack.
* Uses MONGOID_AUTH_MECH: "" for CI, because that mongo database has no user credentials.

Author: pomegranited

.travis/docker-compose-travis.yml

@@ -13,6 +13,8 @@ services:
     container_name: forum_testing
     volumes:
       - ..:/edx/app/forum/cs_comments_service
+    environment:
+      MONGOID_AUTH_MECH: ""
   forum:
     extends: forum-base
     command: tail -f /dev/null

config/mongoid.yml

@@ -9,6 +9,7 @@ common: &default_client
     timeout: <%= ENV['MONGOID_TIMEOUT'] || 0.5 %>
     ssl: <%= ENV['MONGOID_USE_SSL'] || false %>
     auth_source: <%= ENV['MONGOID_AUTH_SOURCE'] || '' %>
+    auth_mech: <%= ENV['MONGOID_AUTH_MECH'].nil? ? ':scram' : ENV['MONGOID_AUTH_MECH'] %>
 
 common_uri: &default_uri
   uri: <%= ENV['MONGOHQ_URL'] %>