common: add missing keywords

In commit d2f9ce0cc14efd02cbab0ff1c736e0764104d771, a new keyword was
introduced : "global_timeout".

But it can't actually be specified in munin-node.conf.

This commit also adds "timeout" in the master and "spooldir" in the node
common: refactor the keywords to ease changes

Using one keyword per line, ordered alphabetically enables a very easy change
tracability
Merge branch 'missing-configuration-keywords' into stable-2.0