Credential Gem: LoginScanner - vmauthd_login

Refactor vmauthd_login.rb to support the Metasploit Credential gem

**Verification**

- [ ] check out the metasploit-credential branch at rapid7/metasploit-credential#68
- [ ] point your gemfile for credential to your local path
- [ ] ````msfconsole````
- [ ] ````use auxiliary/scanner/vmware/vmauthd_login````
- [ ] ````set RHOSTS ,<vmware_host>````
- [ ] ````set USERNAME root````
- [ ] ````set PASSWORD <valid_password>````
- [ ] ````run````
     - [ ]  Scan results should be printed
- [ ]  ````creds````
- [ ]  __VERIFY__ that credentials identified in the scan output are found in the results from creds

Repeat the process using USER_FILE, PASS_FILE, etc.