Initial code to send rebuild requests to Bob on Errata advisory state change.
This adds new handlers.bob.RebuildImagesOnImageAdvisoryChange handler which
handles the ErrataAdvisoryStateChangedEvent.
This handler uses Errata Tool and Pulp to get the list of Docker repositories
in which the images attached to advisory will end up (ended up). It then
requests Bob to rebuild images depending on them.
This also adds new pulp_docker_* config variables, because Docker images are
stored in different Pulp instance than RPMs.