Add CHANGELOG entry for mongo_3_2 role OPS-2205

CHANGELOG.md

+- Role: mongo_3_2
+  - Added role for mongo 3.2, not yet in use.
+  - Removed MONGO_CLUSTERED variable. In this role mongo replication is always configured, even if there is only one node.
+
 - Role: edxapp
   - Added creation of enterprise_worker user to provisioning. This user is used by the edx-enterprise package when making API requests to Open edX IDAs.