Originally, freshmaker skips non-RPM advisory by checking .tar.gz
extension in the builds. This way does not work in all cases. This
patch checks advisory content_types instead. String "rpm" will be
included in content_types if an erratum is a RPM advisory, otherwise
skip that advisory.

• ErrataAdvisoryRPMsSignedHandler._find_images_to_rebuild is updated.
  It does not check if the advisory being handled currently is a RPM
  advisory. This patch ensures only RPM advisory is handled.
• ErrataAdvisoryStateChangedHandler.can_handle is updated. Only handle
  RPM advisory by checking if "rpm" exists in advisory content_types.
• ErrataAdvisoryStateChangedEvent has a new attribute content_types
  to hold value of advisory content_types.
• Manual trigger endpoint is updated as well. Stop and return error in
  response if advisory of given errata ID is not a RPM advisory.
• Tests are updated and added.

Signed-off-by: Chenxiong Qi cqi@redhat.com