glance clear should prompt before deleting all images

[Dan Prince]

Running a 'glance clear' command will delete all of your glance images without prompting the end user. Certainly a handy command to have around however we might consider prompting the user before deleting all of the glance images.

Likewise all we might do the same for 'glance delete'.

For scripting purposes it might also be nice to have a --force or -y option to automatically skip this prompt. The user would need to then specify this additional option in order to have the 'glance delete' or 'glance clear' commands behave as they currently do.