Build images according to configurable Errata state
    
    * Use whitelist to allow to rebuild by advisory state. For development,
      either running freshmaker in development mode or running tests, states
      REL_PREP, PUSH_READY, IN_PUSH, SHIPPED_LIVE are set.
    * ErrataAdvisoryRPMsSignedHandler.handle is changed. If the advisory
      being handled currently is in SHIPPED_LIVE, do not request comopse
      from ODCS for it and start to rebuild the first batch images
      immediately.
    * Add new property errata_state to event class
      ErrataAdvisoryRPMsSignedEvent. It is used in
      ErrataAdvisoryRPMsSignedHandler.handle. And all code that initiate
      ErrataAdvisoryRPMsSignedEvent event object are updated by passing
      advisory.state accordingly.
    * Calling allow_build in BrewSignRPMHandler.handle and
      ErrataAdvisoryStateChangedHandler.handle by passing advisory state to
      determine whether the state is allowed by whitelist.
    * Add tests.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>