The health check requests from haproxy cause uwsgi to write a
lot of useless log lines. This can make it more difficult to find
a problem with a particular service.
This patch adds a route to look for the `osa-haproxy-healthcheck`
user agent string, which haproxy uses when performing health checks.
Any requests with that user agent are not logged.
Closes-Bug: 1742718
Change-Id: I2a1786e2197fbb0fa77e78b94e0c5ff7f4afc8fb
e5e0aa48e3