Add trivial yaml config editor to synctl

[dbkr]

This would make automated synapse installs like docker and unit tests a lot less painful. It keeps comments and formatting, although does change some minor things like the quoting style. Thoughts?

[erikjohnston]

I would prefer if things like this weren't in synctl. I've recently added some stuff to synapse/config/main.py, so maybe that would be a better place for it?

How does this interact with multiple config files?

[dbkr]

It will only edit homeserver.yaml, since synctl is hardcoded to only use this anyway. This seemed like an easy solution that kept it out of synapse core but if there is support in the core for something which makes automated editing/production of config files possible without sed/awk then I'm happy.