Extend glance retries to show() as well

Fixes bug 930245

nova/image/glance.py would already retry get() requests if there was a
connection error, but didn't do so for show() requests.

Change-Id: Ifb1a17da18b7e10ddaaec46e124efcc963681c80

nova/exception.py

@@ -196,6 +196,10 @@ class VirtualInterfaceMacAddressException(NovaException):
                 "with unique mac address failed")
 
 
+class GlanceConnectionFailed(NovaException):
+    message = _("Connection to glance failed") + ": %(reason)s"
+
+
 class NotAuthorized(NovaException):
     message = _("Not authorized.")