New command: container-build
    
    Supports two approaches how to build container images:
    
     - Use osbs[1] client library to talk directly with OpenShift builder
     - Use koji-containerbuild[2] plugin in Koji to submit builds which then
       uses osbs on the Koji builder to talk to OpenShift builder
    
    In the current rpkg implementation user can select the implementation on
    the command line with koji as default. rpkg clients can override this
    behaviour and use their preferred implementation.
    
    Introduces new Require and (because of man page generation)
    BuildRequire: osbs
    
    [1] https://github.com/DBuildService/osbs/
    [2] https://github.com/release-engineering/koji-containerbuild