gardenctl should not be hardcoded

[holgerkoser]

What happened:
If the gardenctl binary is renamed this is not reflected in help, templates, ...

What you expected to happen:
Use os.Args[0] instead of hardcoded name gardenctl

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know:

Environment:

[tedteng]

a heads-up. one scenario I meet today.

If gardenctl is not hard coded and replaced by os.Args[0]. it will trigger the diff error when make gen-markdown && make test because the os.Args[0] randomly generated the main command in the markdown.

< $ eval "$(/var/folders/2v/2qyfy_h52_70zp48y_8whzdw0000gn/T/go-build1097728321/b001/exe/markdown history-env zsh)"
---
> $ eval "$(/var/folders/2v/2qyfy_h52_70zp48y_8whzdw0000gn/T/go-build3323316227/b001/exe/markdown history-env zsh)"