Add a key API to mock_cloud

https://github.com/iotile/python_iotile_cloud/blob/master/iotile_cloud/utils/mock_cloud.py#L109

Specifically, I'd like to run a unit test that exercises this code:

```
archtool = archbuild.arch.ArchTools()
try:
archtool.set_key(self._uuid, "SSH", password, True)
except ExternalError:
raise ExternalError("Setting password in cloud failed",
suggestion="Get admin to delete old password or use set_password to pass the old password")
```