Add headers to usage.rst to make config headings linkable
Add headers to usage.rst to make config headings linkable

- Config options described in usage.rst are now linkable.

Change-Id: Ibe918b28d45d0b08b36af4af89f146d0f9930131
Partial-Bug: #1673530

openstack/collectd-ceilometer-plugin,openstack/collectd-ceilometer-plugin