node: extra 'cap' subarg to initial config

Adding a 'cap' sub-argument to 'config' to signal 2.1+ plugins that
they don't need to do any expensive stuff, they are only called for
the initial node config
node: implement "config cap" in SNMP.pm
node: add "config cap" support to munin-run
node: fix the debug message for "config cap"
plugins: fix the params handling