[SPARK-4155] Consolidate usages of <driver>

We use "\<driver\>" everywhere. Let's not do that.
Why does the local mode use `localhost` in the first place?  I think this takes place in LocalExecutorBackend, where it explicitly uses "localhost" instead of "<driver>".  Could we just change that file to use "<driver>" instead and eliminate the need for this `formatExecutorId` method?