Add configuration flag to set if RecursionDesired should be set on health checkers in Forward-plugin (#3679)

* Make the RD-flag in health-checks in the Forward-plugin configurable

Introduces a new configuration flag; `health_check_non_recursive`. This
flag makes the health-checker do non-recursive requests when checking
the health of upstream servers.
Signed-off-by: Geir Haugom <ghagit@haugom.org>
Signed-off-by: Christian Tryti <ctryti@gmail.com>

* Changes after feedback from reviewer

* Better tests of health-checks with and without recursion
* Removed the health_check_non_recursive configuration in favor of
extending the existing health_check configuration. Now supports an
optional `no_rec` argument.
Signed-off-by: Christian Tryti <ctryti@gmail.com>

* Add new test that checks setup of health_check.
Signed-off-by: Christian Tryti <ctryti@gmail.com>