Feature/appletv login refactor

This PR refactors the appletv_login scanner submitted in https://github.com/rapid7/metasploit-framework/pull/3613

That PR needs to be landed first.

VERIFICATION STEPS
- [ ] set an AppleTV to use the password `foobar`
- [ ] `msfconsole`
- [ ] `use auxiliary/scanner/http/appletv_login`
- [ ] set RHOSTS to point to your appletv
- [ ] `echo "msfadmin\npassword\nfoobar" > /tmp/passes.txt`
- [ ] `set PASS_FILE '/tmp/passes.txt'`
- [ ] `run`
- [ ] VERIFY you get a successful login
- [ ] VERIFY the cred shows up in the creds table
- [ ] set APPLE TV to screen code
- [ ] `set PASS_FILE ''`
- [ ] run
- [ ] VERIFY it bruteforces the code
- [ ] VERIFY the unlock code shows up in the creds table