Nodes and roles -- terminology articles

Create/improve articles for Nodes, Roles, plus one for each
Role that Fuel recognizes.

We have requests for more detailed information about how Fuel
manages nodes and we also have requests for more accessible
information about which roles can and cannot be combined. This
CR does not completely solve the second request but it's a start.

Add ref-arch/logical-setup/0800-node-internals article to give
detailed information about the information Nailgun stores for nodes.

Added tags to ref-arch/logical-setup files so they can be xref'ed

A couple unrelated terminology articles were breaking the build
so I had to add vertical spaces; this is why they are part of this CR.

Change-Id: I76551658e975e2d910686df26c371be142238cc1
Closes-Bug: 1371626
Closes-Bug: 1385141
Merge "Nodes and roles -- terminology articles"