Add new enrollment and troubleshooting doc sections

Add a new Enrollment section to the install guide, explaining
the process of adding Nodes to Ironic.

Add a minimal Troubleshooting section as well, with some basic info
that can be expanded upon later.

Change-Id: I6299aae2b9e7511c61456a7619b9d4e521e48897
Merge "Add new enrollment and troubleshooting doc sections"