add instructions on how to restore influxdb from a gcp snapshot
add instructions on how to restore influxdb from a gcp snapshot

[finishes #132030219]

Signed-off-by: Jen Spinney <475b118f8db4f150e7ed3b69b35fd7c857f9c9f8@hpe.com>

cloudfoundry-incubator/diego-perf-release,cloudfoundry-incubator/diego-perf-release