validate NSX configuration parameters on Settings tab

- right now it is possible to start deploy with empty NSX settings, we
  must validate settings before deployment starts
- validate NSX username, must be not empty
- validate NSX password, must be not empty
- validate transport zone and L3 gateway UUIDs
- validate NSX endpoints and URL, must be valid IPv4 and valid
  HTTP/HTTPS URL
- while here explicitly specify in comments and warnings that right now
  we support only IPv4 addresses

Change-Id: Ie0ad618122034caf6e4a9433bf76c033e241f369
Closes-bug: #1347682