Registering clients to a proxy

[keichwa]

WebUI: Salt-SSH is used to install venv-salt-minion (or salt-minion if user does not use the default configuration)
Bootstrap script: The shell script installs venv-salt-minion (or salt-minion if user does not use the default configuration)
Manual: user has to install venv-salt-minion (hard, where to take it from? it's not in the OS) or salt-minion (in practice this only makes sense for SLE as we don't support Salt from other OSes)

We are considering just automatically migrating any client running the non-Salt Bundle minion to the Salt Bundle minion. If we do that, we don't care too much what salt-minion is used during bootstrapping as we'd change it on a highstate afterwards.

Originally posted by @agraul in #2540 (comment)