Bring the plugin back alive to a working state

[divadsn]

Previously the plugin has failed to deploy giving each time a "unknown command" with the hostname as error output. This PR solves that issue by using another command syntax which was tested carefully to work with FTP and FTPS servers.

I also moved the username option to be a secret instead, to increase the security and minimize a possible bruteforce attack by just hitting the server with a known username.

[cschlosser]

Hey! While it does still work for me what you did looks reasonable so I will use it. Thanks for the username secret!

[divadsn]

Thank you, now it works fine again on Drone CI :) https://build.codebucket.de/LawnchairLauncher/lawnchair.info/18