Better control over automatic image downloads by python client

python-muranoclient supports automatic image downloading. unfortunately, since name is not unique in glance, it checks hash to find out if image is already present in glance. In case image got updated and hash in imags.lst hasn't — client would try to download the image all the time. This is not a bug in client though, as it has no means to tell that image got updated.

On the other hand we could ask user, whether he or she wants to download the image. In horizon this can be done with a separate step, that would list all the images we're about to download and images with matching names/hashes in glance.
In the CLI this can be done by asking for user input and can be controlled-with a separate CLI-parameter.