Retab and fix PR2321

Hi @juushya,

A new pull request has been needed in order to fix https://github.com/rapid7/metasploit-framework/pull/2321

* Module wasn't working: Hardcoded strings were being used as credentials :? On the other hand, according to my testing the authentication queries must be done against the index.html resource.

```

[+] x.x.x.x:80  - Running ServerTech Sentry Switched CDU
[*] x.x.x.x:80 - Starting login brute force...
[*] x.x.x.x:80  - [1/1] - Trying username:"admn" with password:"admn"
[+] x.x.x.x:80 - SUCCESSFUL LOGIN - "admn":"admn"
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(sentry_cdu_enum) > db_
db_connect        db_export         db_nmap           db_status         
db_disconnect     db_import         db_rebuild_cache  
msf auxiliary(sentry_cdu_enum) > creds 

Credentials
===========

host            port  user  pass  type      active?
----            ----  ----  ----  ----      -------
x.x.x.x  80    admn  admn  password  true

```

* These days a big retab effort has been done in the metasploit-framework in order to align it with the ruby best practices (https://github.com/rapid7/metasploit-framework/wiki/Indentation-Standards). Because of that, your pull request would cause conflict. This pull request also fix the Tabulation issue on your pull request, so it can be merged without conflicts. 

Please feel free to test the module after changes. If works, please sends us a pcap of the module working to msfdev[at]metasploit.com in order to verify. Feel free to ask about pgp keys if you need it. Really thanks!