[r=axwalk] HA: upgrade upstart and initiate replset
The machine agent will now upgrade the
Mongo upstart script and initiate the
replicaset when upgrading from a version
prior to 1.19.0.
Tested upgrading 1.18.1 to 1.19.1 with and
without HA enabled; tested straight bootstrap
to 1.19.1 with and without HA enabled.
(Supersedes https://codereview.appspot.com/88350043/)
https://codereview.appspot.com/88790043/