Minor cleanup for f5_bigip_fileaccess

Hi @viable-hartman,

This pull request tries to do final cleanup for https://github.com/rapid7/metasploit-framework/pull/2144. Feel free to review, ask, discuss, check, test etc. the changes, and once you feel comfortable land into your repository. The orginal pull request, https://github.com/rapid7/metasploit-framework/pull/2144, will be automatically updated and hopefully ready to go!

Since we haven't access to a vulnerable device, we should need proof of module working after these last modifications. Normally a pcap makes the thing. But the module is using SSL, not sure if can be disabled in the device in order to capture a session. If no other method of getting a capture of the plain communications will be needed.

Thanks!

juan