Tidy up nova.image.glance
Tidy up nova.image.glance

* Add comment explaining relevance of _is_image_available
* Drop superfluous log statements
* Drop GlanceImageService.delete_all as it has never been implemented
* Drop the 'features' kwarg for GlanceImageService.update
* Code simplification

Change-Id: Ie538dd93dec729fb060bbcddfbddcba6cf303e52

n0ano/ganttclient