SSH proxy: allow using a bare hostname without root@

[jeremy]

Otherwise we can't connect to the proxy as the local user and we can't use ~/.ssh/config to set User directives.

Defaulting to root@ is hard to deprecate without introducing new config. A clean break is probably clearest.

[djmb]

@jeremy - I'll get the pending changes in master out in an release, and then release this on its own so the behaviour change isn't lost in a sea of release notes.