ssh add option to specify the worker username for the ssh command

[petersutter]

What would you like to be added:
Add an option for gardenctl in order to override the username for the shoot worker. E.g. --node-username=admin

Why is this needed:
If the execute cloud config downloader script did not run through successfully, the gardener user is most likely not created and thus a login with this user will fail. In this case the user should try with the admin user as fallback.

[petersutter]

related #315

[tedteng]

one question does the admin user only create on OpenStack as the default user?
Recently I was looking for a feasible solution on this ticket starting from GCP. I checked cat /etc/passwd don't find the admin user. and most of the existing user from os is /usr/sbin/nologin which is unable to use as ssh account. or the admin name is just a substitute in here, could be any name that we created later?

[tedteng]

/assign

[tedteng]

The solution I researched is using gcloud/az cli etc to ssh VM, however, it is not very applicable, and it has a lot of limitations due to permission. /unassign it.

[petersutter]

just for the record: the newly created PR gardener/gardener#9077 should help to ensure that there is a gardener user present