Standardize file permissions, allow non-root

[juniorsysadmin]

This change allows .conf and .load file permission to be changed from a
single place, and potentially helps with non-root installations.

• Change root_group parameter to config_group
• Add config_owner and config_mode parameters
• collectd::plugin::filter::chain Change default permissions for the
  conf file from 0644 to 0640
• collectd::plugin::perl Replace missing root_user with config_owner
• collectd::plugin::perl Change default permissions for the
  configuration directory from 0755 to 0750
• collectd::plugin::perl::plugin Change default permissions for the
  conf file from 0644 to 0640
• collectd::plugin::python Change default permissions for the
  Python module directories from 0755 to 0750
• collectd::plugin::tail::file change default conf file permissions
  from 0644 to 0640
• collectd::typesdb Add config_mode and config_owner parameters

Fixes #719

[juniorsysadmin]

Doc and test updates to be done later in another PR

[bastelfreak]

Should we really do the doc update in another PR? Do we need to adjust tests?

[juniorsysadmin]

Should we really do the doc update in another PR? Do we need to adjust tests?

root_group is not documented at the moment, nor explicitly tested. I would prefer to do separate additions to all of our tests after further some major changes are done shortly, then a separate doc-update pass. Unless the changes look quite risky with high probability of breaking something...