image-pruning: derefence imagestreamtags

Create strong references to images for each pod/bc/dc/etc that uses
<host>/<repo>:tag reference.

Resolves [bz#1498604](https://bugzilla.redhat.com/show_bug.cgi?id=1498604)

Signed-off-by: Michal Minář <[email protected]>

Author: Michal Minář