M$ SCVMM in mgmt_system

Working on it, seems it will be quite easy to do it. There is just only one big drawback - pywinrm cannot make domain user login with simple auth so it has to be done inside of the script which seems to be tremendously slow.
If cred isn't a `SCVMMProvider.Credential`, this will cause an AttributeError.

I can come up with a few possible solutions:
- This should be a hasattr check
- fill should dispatch on `SCVMMProvider.Credential` to handle the domain and then call _fill_credential
- `Provider.Credential` should have a domain attr
- something I haven't thought of