[ansible] Add unixsock to config_files role

[ansible] Add unixsock to config_files role

The unixsock plugin is useful for debugging issues in collectd, and can
be used to verify that metrics are being collected without having to
create CSV files or log into the container.

Typically, the unixsock plugin can be queried using the following
syntax::

$ docker exec bar-collectd /opt/collectd/bin/collectdctl -s /var/run/collectd-socket listval

Signed-off-by: Emma Foley <efoley@redhat.com>
Change-Id: Id342fe7659c809f4eec71085ae6572f9d855684c