Added new boolean column "os_hidden" in images table. Images where
"os_hidden" = True will be omitted from the image list presented
to the user. This will apply to all image visibilities. However,
the images will continue to be discoverable. User can use
filter "os_hidden=true" in GET v2/images call to see all hidden
images.
Implements: blueprint hidden-images
Change-Id: If8f02ca94fdb8e1ac7a81853cd392988900172d1
Abhishek Kekane