MEDIUM: map: uses HAProxy facilities to store default value

MEDIUM: map: uses HAProxy facilities to store default value

The default value is stored in a special struct that describe the
map. This default value is parsed with special parser. This is
useless because HAProxy provides a space to store the default
value (the args) and HAProxy provides also standard parser for
the input types (args again).

This patch remove this special storage and replace it by an argument.
In other way the args of maps are declared as the expected type in
place of strings.