It is likely that there are other commands that we
would like to run in our tests, so this is a new SSH
command runner that the existing SSH check has been
updated to use.
resource_removed is used in many places to monitor removal of
different kinds of OpenStack resource. It is reasonable to expect a
flavor to be removed in a few seconds where as an image based instance
may take minutes to be removed. With that in mind this change
exposes the retry options used by tenacity allowing the caller to
set reasonable expectations for the resource removal.
