Better naming for `config` and `params` in VM and SaaS-related services

[motus]

It is very confusing to have two dicts for config and params as arguments to many methods in, e.g., AzureVMService and AzureConfigService. We should be more explicit in naming and explain the difference between these arguments in the docstrings.

[bpkroth]

One instance of this mismatch: in remote_exec() we take config but in start_host() we take params.

• [ ] Both should probably be params