Refactor the desktopcentral_file_upload exploit

- Rewrite check method
- Declare that v7 is also exploitable (tested and it works)
- Rename to dc_agentlogupload_file_upload to match the other DC module's naming convention
- Add CVE / OSVDB / Full disclosure references

See https://github.com/rapid7/metasploit-framework/pull/3731 and https://github.com/rapid7/metasploit-framework/pull/3741 for background info.