Comment 1 for bug 1489386

Overall, I think the behaviour should be:

glance --os-image-api-version 1 ... -> fail if version 1 is not available
glance --os-image-api-version 2 ... -> fail if version 2 is not available
glance ... -> The user has expressed no preference. Query the endpoint to see if v2 is available -- if so, use it. If not, is v1 available? If so, use it.

It's important that if a user explicitly selects an api version that we fail if that version is not available.

For example an operator may have monitoring scripts such as:

script 1:
# ensure v1 is responsive
glance --os-image-api-version 1 ...

script 2:
#ensure v2 is responsive
glance --os-image-api-version 2 ...

if 'script 2' works by falling back to v1 in this case there's no way for the operator to use the client cli to verify v2 is alive.