node: set the global timeout to 15 min

Commit b15678a0475d64cd63c167b26b7ff61ee42288bb fixed the timeout code. But it
had a nasty side-effect as the timeout setting is _shared_ for global timeout
and plugin timout.

This commit decouples both by introducing a new global_timeout munin-node.conf
variable that defaults to 15 min.
node: add timeout documentation in sample conf