Optimize away most of the path_exists checks
Add logging around failure to read optional/mandatory paths
Just use os.path.exists directly
Log a warning when unable to fetch the openstack versions
Show the available versions in the debug log message
Fixed more of the slowness around fetching and retrying
Fix retry cb to reflect what used to exist
no functional changes, but some minor changes.
Openstack: do not load urls twice

The metadata service openstack implementation would end up fetching
urls more than once, as _path_exists would end up doing a GET.
Now instead, get things you expect to be there.