vault: configure a max elapsed time of 0
vault: configure a max elapsed time of 0

turns out there's a default max elapsed time of 15 minutes. this
resulted in `NextBackOff` returning `backoff.Stop`, i.e. 0, leading to a
tight loop of auth attempts after 15 minutes.

Signed-off-by: Alex Suraci <925ba519bbf1a06224928b692ef88b93aee946af@gmail.com>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse