MEDIUM: bwlim: Support constants limit or period on set-bandwidth-limit actions

MEDIUM: bwlim: Support constants limit or period on set-bandwidth-limit actions

It is now possible to set a constant for the limit or period parameters on a
set-bandwidth-limit actions. The limit must follow the HAProxy size format
and is expressed in bytes. The period must follow the HAProxy time format
and is expressed in milliseconds. Of course, it is still possible to use
sample expressions instead.

The documentation was updated accordingly.

It is not really a bug. Only exemples were written this way in the
documentation. But it could be good to backport this change in 2.7.