[MINOR] http: make the "HTTP 200" status code configurable.

[MINOR] http: make the "HTTP 200" status code configurable.

This status code is used in response to requests matching "monitor-uri".
Some users need to adjust it to fit their needs (eg: make some strings
appear there). As it's already defined as a chunked string and used
exactly like other status codes, it makes sense to make it configurable
with the usual "errorfile", "errorloc", ...