Allow customization of Paramiko's host-key verification policy.

My organization's security policies require us to whitelist remote SSH keys, so making the policy configurable at all was a required change from stock dulwich.

This pull request also changes the default host-key verification policy from the do-nothing "[MissingHostKeyPolicy](https://github.com/paramiko/paramiko/blob/master/paramiko/client.py#L463)" to "[WarningPolicy](https://github.com/paramiko/paramiko/blob/master/paramiko/client.py#L510)" which *will* result in a behavior change: a warning will be printed using the standard Python "warnings" module upon connecting to an SSH server whose public key fingerprint is not in `~/.ssh/known_keys`. I personally think this is a good thing - key verification is a fundamental component of the SSH protocol - but I can revert this particular change if you think this will just annoy most dulwich users.