Kerberos SSO authentication

Implements basic kerberos SSO authentication.
Both check-realms and check-login will now boot a ipa machine, but only one can be running at any one time.  (Because they both try to be on the same network with the same IP address.)  check-verify will execute test cases in parallel, and if both check-login and check-realms are running at the same time, one of them will fail like this:

```
Failure: Mac address 52:54:00:9e:00:F0 is in use
```