Switch to use update_config instead of save_config in Config.update
function to ensure charm configs are updated and not overwritten.
Fix wiring lines in _save_config function.
Raise ApplyError if grub-mkconfig command failed, since it is failing
also if there is wrong validation.

Since the grub lib required to run integration tests on machine or VM,
we are switch to running integration tests directly on GitHub runner.
It was not possible to run it on VMs, since the GitHub runner do not
supports nested virtualization.