Use `username' in ImagesConfig

Use username key as defined in config example.

This fixes bug #978932

Change-Id: Ieff714f1dcbd8502d3f8bb0885371d4e6ff3064f
Signed-off-by: Julien Danjou <julien@danjou.info>

tempest/config.py

@@ -217,7 +217,7 @@ class ImagesConfig(BaseConfig):
     @property
     def username(self):
         """Username to use for Images API requests. Defaults to 'demo'."""
-        return self.get("user", "demo")
+        return self.get("username", "demo")
 
     @property
     def password(self):