Can't create a metadata named key, clear, delete

We now pass metadata as a dictionnary and not as keyword arguments since
_metadata method already have 3 keywords arguments (key, clear and delete)
which couldn't be used as metadata keys.

Change-Id: Iee3773e4e9a4187bd1557f400aa966f80e3acb3c
Story: 2005085
Task: 35787

openstack/compute/v2/metadata.py

@@ -19,7 +19,8 @@ from openstack import utils
 class MetadataMixin(object):
 
     def _metadata(self, method, key=None, clear=False, delete=False,
-                  **metadata):
+                  metadata=None):
+        metadata = metadata or {}
         for k, v in metadata.items():
             if not isinstance(v, six.string_types):
                 raise ValueError("The value for %s (%s) must be "
@@ -84,7 +85,7 @@ class MetadataMixin(object):
         if not metadata:
             return dict()
 
-        result = self._metadata(session.post, **metadata)
+        result = self._metadata(session.post, metadata=metadata)
         return result["metadata"]
 
     def delete_metadata(self, session, keys):

releasenotes/notes/metadata-key-name-bugfix-77612a825c5145d7.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - Fixed a bug related to metadata's key name. An exception was
+    raised when setting it to "delete"," clear" or "key"