Changes

image-pruner: Determine protocol just once
Determine the registry protocol once. Do not change to other protocol 
during the run. This will produce nicer output without unrelated 
protocol fallback errors.
Do not default to insecure connection when the --registry-url is empty.
Move registry client initialization just before the start of the pruner
- so we can precisely determine whether to allow for insecure fall-back 
based on collected images and image streams.
Move ping() outside of pruner. Instead, determine the registry URL 
before the pruner starts and assume it won't change during the run.
Signed-off-by: Michal Minář <miminar@redhat.com>
 (commit: ce7072f)

image-pruner: Reenable registry-url validation
Signed-off-by: Michal Minář <miminar@redhat.com>
 (commit: 7fd22bb)