worker: behaviour: parse --garden-max-containers as string
worker: behaviour: parse --garden-max-containers as string

k8s topgun was failing because the value for
`CONCOURSE_GARDEN_MAX_CONTAINERS` was quoted, while the flag was a
`*int`. Let's just unmarshal it as a string, pass it on to guardian, and
allow guardian to do the validation.

Signed-off-by: Aidan Oldershaw <30ef7a6e0d016bc846f26d89dacbd23aceded206@pivotal.io>

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