Copy of security guide from openstack-manuals

This is a direct copy of the security guide as it stands today in the
openstack-manuals repositry.  This commit will serve to move the guide
into this repository.  Once this is complete, we can remove the
security guide from openstack-manuals.

Change-Id: I2be2db618b3bb9fc4b03ee97ca8d48f75cb2f507