Clean up for sap_soap_rfc_pfl_check_os_file_existence

Hi @nmonkee,

In this pull request I'm trying to cleanup sap_soap_rfc_pfl_check_os_file_existence Please feel free to review, discuss, test, etc any of the changes proposed. Once you feel comfortable just land this pull request in your repository and the original one: https://github.com/rapid7/metasploit-framework/pull/1658 will be automatically updated. And we'll be ready to merge it.

Also, you might want to check our docs on landing pull requests, https://github.com/rapid7/metasploit-framework/wiki/Landing-Pull-Requests , and would be nice if you have into account the benefits of clean, short, informative merge commit messages:  git merge --no-ff --edit and sticking to the usual 50/72 formatting for commits makes for pleasantly readable logs.

Don't hesitate to ask any doubts and, also, if while we work in this modules you feel comfortable enough and have the time to incorporate these recommendations to the existing sap pull requests would accelerate things for sure :-) Anyway will keep helping with these pull requests! 

Summary of Changes:

* Keep modules filenames shorter.
* Run tools/msftidy.rb, modules must be compliant.
* Use the guidelines proposed on https://github.com/rapid7/metasploit-framework/wiki/How-to-Send-an-HTTP-Request-Using-HTTPClient when using HTTPClient.
* Since it's an Scanner module, in order to avoid verbosity, try to use print only for the minor information possible (normally success). For the others prints use the verbose print => vprint_*
* Please add the name of the vulnerability discoverer to the Authors section when available, helps to document modules.
* Please add known references such as OSVDB, BID or CVE to the References section when available, helps to document modules.
* When there are required options always make things easier to provide a default option if possible.
* Parsing profitable information, in this case the dir enumeration from the xml response helps to present better the results. Even when storing the loot is a good thing to store the full response. Even when int his Scanner module the parsed results are only showed to the user when mode VERBOSE = true.


Thanks!