Provide consistent spelling of the microversion header

Headers must not be case-sensitive, but since we use a simple dict for
collecting them, it's possible to end up with two conflicting headers
for the API version. Make sure to use the same spelling.

Change-Id: I9c9554e4d317c7d06f9c78162967218fbf118b50
diff --git a/ironic_tempest_plugin/services/baremetal/v1/json/baremetal_client.py b/ironic_tempest_plugin/services/baremetal/v1/json/baremetal_client.py
index d8f1cfa..be1262d 100644
--- a/ironic_tempest_plugin/services/baremetal/v1/json/baremetal_client.py
+++ b/ironic_tempest_plugin/services/baremetal/v1/json/baremetal_client.py
@@ -40,8 +40,8 @@
         for iface in base.SUPPORTED_INTERFACES
     )
 
-    @staticmethod
-    def _get_headers(api_version):
+    @classmethod
+    def _get_headers(cls, api_version):
         """Return headers for a request.
 
         Currently supports a header specifying the API version to use.
@@ -55,7 +55,7 @@
         headers = None
         if api_version is not None:
             extra_headers = True
-            headers = {'x-openstack-ironic-api-version': api_version}
+            headers = {cls.api_microversion_header_name: api_version}
         return extra_headers, headers
 
     @base.handle_errors
@@ -839,7 +839,7 @@
         headers = None
         if api_version is not None:
             extra_headers = True
-            headers = {'x-openstack-ironic-api-version': api_version}
+            headers = {self.api_microversion_header_name: api_version}
         return self._list_request('nodes/%s/vifs' % node_uuid,
                                   headers=headers,
                                   extra_headers=extra_headers)