Add new gitlab-workhorse config settings

  -proxyHeadersTimeout duration
    	How long to wait for response headers when proxying the request (default 1m0s)

The default behavior should be not to pass this setting at all. Only if node['gitlab']['gitlab-workhorse']['proxy_headers_timeout'] is a non-empty string (or int) should we add the switch.

cc @dblessing @marin

@marin this should go in 8.4; it is something we forgot in 8.3.

Milestone changed to 8.4

mentioned in merge request !610 (merged)

Status changed to closed by merge request !610 (merged)

mentioned in commit d3de62c5