schema: Make cloud organization optional

[radeksimko]

As reported in hashicorp/vscode-terraform#1577 the organization attribute is in fact optional.

The documentation is somewhat confusing in that it states that it both organization and workspaces are required fields

but further down it basically says that it's not, implying that it can be supplied via env variables

I suppose it's still technically "required" in the sense it has to come from somewhere but it's not required in the configuration.

[radeksimko]

Just for posterity, I did check the equivalent field in the remote backend and that one still appears to be required, which I think makes sense, given that there will always likely be more than one data source in one module, unlike with a backend.

[jpogran]