Make Cisco SSL VPN Privilege Escalation landable

Hello @claudijd,

This pull request tries to clean https://github.com/rapid7/metasploit-framework/pull/3305 and hopefully finish it. It includes:

* Solve conflicts to make it landable. It includes rebasing from rapid7/master. Because of that the pull request shows all these changes. S
* Clean modules/auxiliary/scanner/http/cisco_ssl_vpn_priv_esc.rb

Unfortunately I can't test the module and the last changes. If the changes work for you and you could just ad a copy and paste of the console with the module working, it is enough for me to go.

Thanks.