Adds the leader-elector container support that was removed in
PR #568. The new vault-k8s uses an internal mechanism for leader
determination, so this is just for backwards compatibility, and can
be removed in the near future.
* mark the endpoint as deprecated
* add a new useContainer option for leaderElector
Default to not deploying the old leader-elector container, unless
injector.leaderElector.useContainer is `true`.
