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
diff --git a/tempest/api/compute/images/test_image_metadata.py b/tempest/api/compute/images/test_image_metadata.py
index b497626..1f3af5f 100644
--- a/tempest/api/compute/images/test_image_metadata.py
+++ b/tempest/api/compute/images/test_image_metadata.py
@@ -28,6 +28,7 @@
 
 
 class ImagesMetadataTestJSON(base.BaseV2ComputeTest):
+    max_microversion = '2.38'
 
     @classmethod
     def skip_checks(cls):
diff --git a/tempest/api/compute/images/test_image_metadata_negative.py b/tempest/api/compute/images/test_image_metadata_negative.py
index 03d0789..407fb08 100644
--- a/tempest/api/compute/images/test_image_metadata_negative.py
+++ b/tempest/api/compute/images/test_image_metadata_negative.py
@@ -20,6 +20,7 @@
 
 
 class ImagesMetadataNegativeTestJSON(base.BaseV2ComputeTest):
+    max_microversion = '2.38'
 
     @classmethod
     def setup_clients(cls):