MINOR: sample: provide a generic var-to-sample conversion function

MINOR: sample: provide a generic var-to-sample conversion function

We're using variable-to-sample conversion at least 4 times in the code,
two of which are bogus. Let's introduce a generic conversion function
that performs the required checks.

(cherry picked from commit 168e8de1d06adc7aa3e7e2cc2a36935a77c79b9c)
[cf: call to vars_get_by_desc() was updated to remove the third param]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit ea5735057fcf62c74b28fe3f13a0ab96b8014852)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>