Added AUTHORIZED_KEYS field into settings.yaml.erb

This is for making it possible for nailgun to pass
default (aka system) public key into provisioning data
so as to put them on a target node apart from user
defined public key.

Related PR to fuel-web is
https://review.openstack.org/#/c/127490/

Change-Id: Id5dff61444d7dfb278e0ae6c49b0f772ff68ffd6
Closes-Bug: 1378832
Implements: blueprint image-based-provisioning
Merge "Added AUTHORIZED_KEYS field into settings.yaml.erb"