collectd: Clarify config file example/placeholder

collectd: Clarify config file example/placeholder

The config file shipped with collectd dates from 2010 and
leads to error messages if luci-statistics & collectd are installed,
as it references several plugins not usually installed, or such ones
that have been renamed since then.

For most users, this file is just a placeholder during collectd installation,
as /usr/bin/stat-genconfig from /etc/init.d/luci_statistics will overwrite it.

Sanitize and shorten the placeholder config file:
* Reference actively only the default plugins installed by luci statistics.
* Match the placeholder config with the genuine config from luci statistics.

If somebody uses collectd separately from luci statistics, he will need to
edit these settings anyway.

Signed-off-by: Hannu Nyman <redacted>