Don't default to standard HTTP check and allow custom URL to query

[bvansomeren]

The role by default just adds a "check" to each server, however just loading the / url may give errors (403, 404 etc) causing the backend to fail. Also we might want to check an API endpoint.

We should implement:

• Default remains just a simple check to preserve old deployments
• Allow disabling of the check per backend
• Allow setting a custom URL to check per backend

[bvansomeren]

More documentation:

https://www.haproxy.com/blog/using-haproxy-as-an-api-gateway-part-3-health-checks/

Basically support adding:

option httpchk GET /check HTTP/1.1\r\nHost:\ mywebsite.com

Many deployments require HTTP 1.1 and host headers so might as well make that the default