Move client generation to CLI methods

[SchoolGuy]

This is needed so when executing --help or generating the shell completion no login is attempted to be performed.

Fixes #42

Building the project can be now done without the exit status 1:

enno@dell-laptop ~/S/G/cobbler-cli (main)> make build
building package
go build -o cobbler main.go
creating shell completions
make shell_completions
make[1]: Verzeichnis „/home/enno/Sources/GolandProjects/cobbler-cli“ wird betreten
./cobbler completion bash > config/completions/bash/cobbler
./cobbler completion fish > config/completions/fish/cobbler
./cobbler completion powershell > config/completions/powershell/cobbler
./cobbler completion zsh > config/completions/zsh/cobbler
make[1]: Verzeichnis „/home/enno/Sources/GolandProjects/cobbler-cli“ wird verlassen

[tiltingpenguin]

LGTM