Increased number of 500 errors during git clones over https

Graphs:

• fleet overview during the time: https://performance.gitlab.net/dashboard/db/fleet-overview?orgId=1&from=1508189217392&to=1508190950830
• git nodes closeup: https://performance.gitlab.net/dashboard/db/fleet-overview?refresh=5m&panelId=35&fullscreen&orgId=1&from=1508189217392&to=1508190950830

Slack discussion start: https://gitlab.slack.com/archives/C101F3796/p1508190528000416

The 21:30 (when 200ies gone to zero) roughly corresponds to deployment to nfs nodes. We need the timestamps in the takeoff log for easier correlation.

/cc @gl-infra