Refactor to store compose id to Compose model
    
    Instead of storing compose ID to Event.compose_id and
    ArtifactBuild.build_args['odcs_pulp_compose_id'], this patch stores
    compose IDs to Compose model and build the relationship to each
    ArtifactBuild.  This is flexible for adding more composes, for example,
    a base image requires a compose containing boot.iso but other images
    don't.
    
    Major changes
    
    * ErrataAdvisoryRPMsSignedHandler._prepare_yum_repo is refactored and
      now it returns the requested new compose instead of storing into
      database directly.
    * ErrataAdvisoryRPMsSignedHandler._record_batches is updated to store
      pulp compose id to Compose instead of
      ArtifactBuild.build_args['odcs_pulp_compose_id'].
    * ContainerBuildHandler.get_repo_urls now simply gathers an
      ArtifactBuild's repo URLs by querying database through
      ArtifactBuildCompose relationship.
    * ContainerBuildHandler._build_first_batch is removed.
    * In ErrataAdvisoryRPMsSignedHandler.handle, call start_to_build_images
      to build first batch directly. It's simple enough, so no need of
      extra call to _build_first_batch.
    * ErrataAdvisoryRPMsSignedHandler._prepare_yum_repos_for_rebuilds is
      updated to store composes into Compose, which are requested for
      current event and dependent events. Original code was to store compose
      id into dependent event's compose_id, that was actually a bug. Now, it
      is fixed.
    * ComposeStateChangeHandler.handle is rewritten. Start to rebuild a
      image, which is in first batch, only when all composes finish. In
      handle method, just call start_to_build_images with builds that can
      be rebuilt.
    * Some helper methods are added to models.
    * Add and update tests as well.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>