Provider netcup

[Azorimor]

This PR adds basic support to use Netcup as provider (#282). The implementation only updates the IP of the given host. Additional settings (TTL, Retry, Refresh, ...) can be set manually in the netcup customercontrolpanel / dashboard.

All requests to the netcup api are send to the same endpoint. The different functions are defined by the body of the request. For example the following requests (and more) are necessary. (see doc) The requests are implemented in netcup/client.go.

Login (Create Session on server, which is valid for 15 minutes (logout not implemented, because 15 min as default seems ok)) body:

"action": "login",
"param": {
  "customernumber": "111",
  "apikey": "xxxxx",
  "apipassword": "yyyy"
}