Allow reboot/shutdown on nodes without IPMI

We have code falling back to ssh commands when IPMI is not there.
Only show identify IPMI button for admin node

Obviously, poweron doesn't make sense. I don't think we want to allow
rebooting/shutting down the node this way, as there's a risk of the task
not completing and therefore breaking some internal consistency.
Merge pull request #697 from vuntz/ipmi-buttons

Always show reboot/shutdown, except for admin node