In order to get puppet out of the business of mucking with exim and
fighting ansible, finish moving the config to ansible.
This introduces a storyboard group that we can use to apply the exim
config across both servers. It also splits the base playbook so that we
can avoid running exim on the backup servers. And we set
purge_apt_sources the same as was set in puppet. We should probably
remove it though, since none of us have any clue why it's here.
Change-Id: I43ee891a9c1beead7f97808208829b01a0a7ced6
0d1f235fce