BUG/MINOR: vars: Fix the set-var and unset-var converters

BUG/MINOR: vars: Fix the set-var and unset-var converters

In commit 3a4bedccc6 the variable logic was changed. Instead of
accessing variables by their name during runtime, the variable tables
are now indexed by a hash of the name. But the set-var and unset-var
converters try to access the correct variable by calculating a hash on
the sample instead of the already calculated variable hash.

It should be backported to 2.5.

(cherry picked from commit bb3e80e1814f63b76f100b342c217d08ca719688)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>