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")
```

Start in iotile_cloud/utils/mock_cloud.py around line 109 and trace the mocked cloud API used by ArchTools.set_key. Add the key-related API needed for a unit test to exercise this path, then verify that the test can reach the set_key call and cover the shown error handling without relying on the real cloud.

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