Fix Incorrect Exception when metadata is over 255 characters

Need to be in sync with Essex. Throw a new exception in api.py when key or value
is over 255 chars. Catch this exception end return back HTTP error 413.

add extra lines in exception.py to make pep8 happy

Fixes bug 1004007

Change-Id: I67694e3a9600b7dd0b80c519327327568751c2a4

nova/exception.py

@@ -263,6 +263,10 @@ class InvalidMetadata(Invalid):
     message = _("Invalid metadata") + ": %(reason)s"
 
 
+class InvalidMetadataSize(Invalid):
+    message = _("Invalid metadata size") + ": %(reason)s"
+
+
 class InvalidPortRange(Invalid):
     message = _("Invalid port range %(from_port)s:%(to_port)s. %(msg)s")