setup-ssh: try to use key auth first
setup-ssh: try to use key auth first

This patch changes the setup-ssh workflow to try key authentication
first, and then fall-back to password authentication. The password is
also read lazily, with no prompts if we can authenticate via keys.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

yiannist/ganeti,yiannist/ganeti,ganeti/ganeti,mbakke/ganeti,dimara/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,apyrgio/ganeti,dimara/ganeti,leshchevds/ganeti,andir/ganeti,grnet/snf-ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/snf-ganeti,ganeti/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,bitemyapp/ganeti,leshchevds/ganeti,andir/ganeti,onponomarev/ganeti,bitemyapp/ganeti,leshchevds/ganeti,yiannist/ganeti,onponomarev/ganeti,mbakke/ganeti,grnet/snf-ganeti,apyrgio/ganeti