Use Remote.hostname in logs

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add Remote.ensure_online()

If the connection is alive, do nothing. If not, reconnect. Allow any
exceptions to bubble up to the caller. This is intended to fix unhelpful
'Bad File Descriptor' errors we were seeing when VMs go down.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>