stack/erp5: rate limit clients on backend haproxy

This supports defining rate limits as maximum number of matched requests
over a period of time for the same client IP.

Requests are matched either by a regular expression on URL path or a
response status code.

Once a client exceeded the rate limit, haproxy will reply with HTTP
status code 429 ("Too Many Requests") until the expiration period
configured on the rule has passed. Optionally, the responses can be made
after the "tarpit" delay.