tests: Make image_copy can work with nfs again

As asset.download_file will only handle urls(unknown url type will be reported)
which makes the nfs mounted to local can not be used for image copy. So use
utils.get_file when we use a nfs mounted in local.

Also fix the error.context_aware missing, update the error image store
path and add the error contexts.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>