Verify the given host key for the given IP and host, asking for confirmation from, and notifying, the given UI about changes to this file.
| Parameters | ui | The user interface to request an IP address from. |
| hostname | The hostname that the user requested to connect to. |
| ip | The string representation of the IP address that is actually being connected to. |
| key | The public key of the server. |
| Returns | a Deferred that fires with True when the key has been verified, or fires with an errback when the key either cannot be verified or has changed. (type: Deferred) |