[RFC] Add a `site.conf` section

[cpb-]

It seems interesting to support the creation of conf/site.conf from a menu.

In the case when the site.conf is present in a menu and we don't want to use it, a --ignore-site command line option for cooker will be needed.

[cpb-]

There are other ways to support the site.confequivalent :

• add a command-line option (--site-specific <file>) that copy the given file into conf/site.conf before the build(s);
• add a site-specific section containing one or more subsections to include into the local.conf file only if specific conditions are validate. The subsection contains a string that is tested (as a shell-command) to know if the given subsection has to be included. Simplest condition strings could be true or false. More interesting conditions would be ping <an-internal-node> to know if proxy rules are needed.