Check API version on 'restart_policy'

[hongbin]

Below is @jtopjian's feedback from Gophercloud. As mentioned by @jtopjian, we should check API version on the 'restart_policy' property.

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@hongbin From reading the upstream Zun patches, if Gophercloud supported microversions, I don't think that would have prevented this issue.

The upstream patch you identified as the cause of the test failures is here: https://review.openstack.org/#/c/571398/

I might be incorrect, but I don't see anything in that code which is checking for a microversion before performing an action. As an example, this is how Nova handles different behavior depending on the microversion:

https://github.com/openstack/nova/blob/master/nova/api/openstack/compute/migrations.py#L116-L143

According to the microversion spec, all prior behavior should still be successful and not require a microversion increase to the client. It's only after the client opts-in to a microversion that new features, such as restart_policy should be seen.
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