Delete unpacked iso on repo deletion

When issuing the `delete-repo` command, the unpacked repo was being left on the filesystem. This change cleans up this folder, if it exists.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-202
Fix retry of commands

If a command failed for a retry-able reason, an item that could not be encoded into EDN was being placed on the queue since it contained a Command object. This fixes that by preserving the original command arguments for re-queuing.