Convert nodes to authenticate using oauth

This involves the following:

Adding an /etc/pulp/nodes configuration file so we can specify the node_id and oauth credentials. For normal pulp
installs, much of this information is either defaulted from the server.conf, consumer.conf or consumer certificate.  However, to support installations that do not include pulp-consumer, users will provide a good bit of this information.  The *resources* module encapsulates this complexity.

In an effort to make this as painless as possible for the user, I changed the server.conf so that oauth is
now enabled by default with oauth (key & secret) generated during install.

Updated pulp.spec to update server.conf with generate values for oauth (key & secret) only when no values are set.

The user-guide *Nodes* section and 2.3 release notes updated.