Drop deprecated functions in buildstream._yaml

There's tons of warnings being spewed
```
round_trip_dump will be removed, use

yaml=YAML()
yaml.dump(...)

instead
_yaml.roundtrip_dump(config, filename)
```
in various bst-plugins-experimental tests because BuildStream is using deprecated API

Start by reading buildstream._yaml and locating the deprecated API uses that produce warnings in the bst-plugins-experimental tests. Run the affected tests to reproduce the warnings; the work is done when those deprecated-function warnings are gone and the tests still pass.

Written by the indexing model from the issue text.