Add validation to check imageContentSources registry is listed in the pull-secret

In a disconnected install the registries listed in imageContentSources should have corresponding entries in the pull-secrets. If the registry does not exist in the pull-secrets, then the "agent create image" command will fail and the error does not indicate that missing pull-secret credentials is the culprit.

We should add validations to check registries listed in imageContentSources have corresponding entries in pull-secret.    

    4.16

    Always

    1. Created a disconnected environment with a mirror registry.
    2. Omit the registries credentials from the pull-secret
    3. Create the agent ISO on a server that is disconnected
    

    Agent installer tries to pull base ISO from public mirror because local mirror is not reachable. Create image command fails because public mirror is also unreachable.

    Error explaining that the registry is missing in the pull-secrets