IMPALA-6959: [DOCS] Update to HAProxy configuration sample code
IMPALA-6959: [DOCS] Update to HAProxy configuration sample code

- Changed to deprecated timeouts: contimeout, clitimeout, srvtimeout
- Changed the sample timeout values to more realistic values
- Added a note that actual timeout values should depend on
  the user cluster

Change-Id: Idff3aa9bbb58c1953cb7e9394ade01c7833c3a34
Reviewed-on: http://gerrit.cloudera.org:8080/10284
Reviewed-by: Alan Choi <91e38e63b890fbb214c8914809fde03c73e7f24d@cloudera.com>
Reviewed-by: Alex Rodoni <b46da68bf9b818cec7052f7cdfe9b3e69485676b@cloudera.com>
Tested-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>

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