Use detail instead of explanation for 415 errors

As of webob 1.5.1 415 exception does not have explanation in it's
template. 'detail' is present in both versions templates.
Also adds CONTENT_TYPE to test-requests environment, to allow tests to
run correctly.

Change-Id: I52f2686ee838eef38ea9c5f48fe5f9e093e82412
Partial-Bug: #1512390

murano/common/wsgi.py

@@ -404,7 +404,7 @@ class Resource(object):
             action, action_args, accept = self.deserialize_request(request)
         except exceptions.UnsupportedContentType:
             msg = _("Unsupported Content-Type")
-            return webob.exc.HTTPUnsupportedMediaType(explanation=msg)
+            return webob.exc.HTTPUnsupportedMediaType(detail=msg)
         except exceptions.MalformedRequestBody:
             msg = _("Malformed request body")
             return webob.exc.HTTPBadRequest(explanation=msg)

murano/tests/unit/api/base.py

@@ -147,6 +147,7 @@ class ControllerTest(object):
             'PATH_INFO': path,
             'wsgi.url_scheme': 'http',
             'QUERY_STRING': '',
+            'CONTENT_TYPE': 'application/json',
         }
 
     def _simple_request(self, path, params=None, method='GET',