Fixes bug 1039675
glanceclient can raise HTTPNotFound (as well as NotFound) it appears,
but glance image service is only converting NotFound ->
InstanceNotFound. Same applies to 'Forbidden' and other exceptions.
This patch converts 'exc_type is NotFound'-like checks to use
'isinstance' instead, which will cover HTTPNotFound, etc.
Also add missing glance/image tests that weren't in Cinder
Change-Id: I8787598daccf401a26d9397946ecd55f89ff62b3