In the case that no advanced.config file exists (everycase!) rt
would not add any advanced config settings to the conf.
This PR teaches rtdev to create an advanced.comfig file if none exists
so that tests may set advanced config.
In this case we set ring_size and also the `crdt_mixed_versions` app env
Still have not completed upgrade and feature-flag switch.
I changed the versions from atoms to "2.0.2" and "2.0.4", we can
bikeshed that with build/test czars on Monday.
Added some useful logging statements to the plain-upgrade test.
Removed unnecessary clean_cluster and systest_read calls.