Employ no-downtime update strategy for apps on Nomad

[Vaughn Dice]

For apps on Nomad, we should look into the possibility of employing a default no-downtime upgrade strategy (regardless of replica count).

E.g. from https://www.nomadproject.io/docs/job-specification#example:

# Specify this job to have rolling updates, two-at-a-time, with
# 30 second intervals.
update {
  stagger      = "30s"
  max_parallel = 2
}

[Matthew Fisher]

related: #890