MINOR: lbprm: support for the "none" hash-type function

MINOR: lbprm: support for the "none" hash-type function

Allow the use of the "none" hash-type function so that the key resulting
from the sample expression is directly used as the hash.

This can be useful to do the hashing manually using available hashing
converters, or even custom ones, and then inform haproxy that it can
directly rely on the sample expression result which is explictly handled
as an integer in this case.