Add ability to skip pruning operations with exiting with an error

Story: As a user of oc-mirror, I want to be able to opt of pruning operations exiting with an error.

Background: 

oc-mirror pruning operations are currently reliant on the contents of the imageset configuration file. Other than managing the imageset configuration in an additive manner, the solution to opt-out of pruning can be done by blocking the action on the registry and using "--continue-on-error" to ignore the Not Allowed errors. This will still attempt the pruning and for users who do not want to utilize the pruning feature, this is wasted operation time.

Acceptance criteria:

• oc-mirror has a flag (possibly skip-prune) to allow pruning operations to be skipped completely.