Rhiza env should be optional

[HarryCampion]

I think the .rhiza.env should be optional and several defaults used if not specified. Ideally users should be able to minimise the upstream rhiza deps.

Another thought would be to allow users to embed env vars in the template file.

[HarryCampion]

Linking janushendersonassetallocation/loman#150

Perhaps template file can have an env vars section.

env_vars:
...

Or

overrides:
...

[HarryCampion]

@markrichardson

[tschm]

Yes, optional .rhiza.env makes sense