Set max_microversion 2.38 for nova image-metadata tests

https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id35

Besides, there's inconsistency between https://developer.openstack.org/api-ref/compute/#create-image-metadata
and https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id35,
https://review.openstack.org/#/c/554813/ will fix it.

Change-Id: Ic6cc22f048c2ccd427dfe6e6aea0273128d96592
2 files changed