minor changes to security documentation
minor changes to security documentation

This contains some minor changes to the security documentation:
- replace 'ironic' with 'the Bare Metal service' (as per documentation
  guidelines
- fixes a few grammatical issues
- removes reference to "clean_nodes" configuration option since it
  has been deleted in Newton
- clarifies that [deploy]/erase_devices_priority cannot be 0 for
  erasing of devices to happen
- added links
- additional references shows reference descriptions instead of the links

Change-Id: I11df3bde9eff4b7f109bee6b9d0058e325b67027

openstack/ironic,NaohiroTamura/ironic,pshchelo/ironic,bacaldwell/ironic,NaohiroTamura/ironic,SauloAislan/ironic,devananda/ironic,SauloAislan/ironic,openstack/ironic,pshchelo/ironic,bacaldwell/ironic