Implement --skip-validate for cli image add

[SebTardif]

This is to finish the epic of supporting an insecure registry, half of the work have been done, but still missing support for the cli action 'image add'. See https://github.com/anchore/anchore-engine/issues/76

[dspalmer99]

Thanks for raising this, @SebTardif . We will review the feature request and get back to you when we have some feedback.

[zhill]

It sounds like the ask is a way to tell Anchore to not require a trusted cert for a registry, regardless of its credential needs. Something like:

anchore-cli registry add <myregistry> --insecure

But in such a way that future operations with would remember to not expect a valid cert.