Opt out of Container API

[RTann]

Let's say I am using Clair in air-gapped mode. Then, I may only reach private, loopback, and link-local unicast addresses.

Let's also say I would like to index an old Red Hat-official image (before content_sets were added). Then, ClairCore will attempt to reach out to the Container API, as this is always configured. Since the HTTP client cannot reach the default Container API, ClairCore will return an error and log a warning.

It would be nice to have the ability to opt out of the Container API and just continue without CPEs.

For example, the StackRox scanner currently opts to not even support the Container API, as we wish to minimize network connections, and we'd rather tell users to use newer Red Hat containers. We would like to be able to have the ability to opt-out of the Container API so we don't waste time trying it nor logging warnings.