MINOR: sample/arg: Be able to resolve args found in defaults sections

MINOR: sample/arg: Be able to resolve args found in defaults sections

It is not yet used but thanks to this patch, it will be possible to resolve
arguments found in defaults sections. However, there is some restrictions:

  * For FE (frontend) or BE (backend) arguments, if the proxy is explicity
    defined, there is no change. But for implicit proxy (not specified), the
    argument points on the default proxy. when a sample fetch using this
    kind of argument is evaluated, the default proxy replaced by the current
    one.

  * For SRV (server) and TAB (stick-table)arguments, the proxy must always
    be specified. Otherwise an error is reported.

This patch is mandatory to support TCP/HTTP rules in defaults sections.