Make fetch_cell_logs script idempotent
Make fetch_cell_logs script idempotent

Signed-off-by: Leo Chu <a4dab1a96fe3d3ca42abb90f3212fe8a512a2bf1@gmail.com>

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