Clarify description string for image metric

This metric seems to have an existential crisis.
Change to more succinct "Image existence check"

Change-Id: I953c55214e73bb93c452b25f97960e9637f1008c
Closes-Bug: #1292937

openstack_dashboard/api/ceilometer.py

@@ -1062,7 +1062,7 @@ class Meters(object):
         return datastructures.SortedDict([
             ('image', {
                 'label': '',
-                'description': _("Image polling -> it (still) exists"),
+                'description': _("Image existence check"),
             }),
             ('image.size', {
                 'label': '',