Minor fixes for the nova section (install guide)

- Spacing/formatting updates
- Set the rpc_backend value to 'rabbit' for debian/ubuntu. This probably
  needs to be done for SUSE/RHEL ('rabbit' or 'qpid') but needs to be
  validated first.
- Simplify some examples
- On Ubuntu, remove the sqlite database before restarting the services

Change-Id: I0469e07d657ccd6a183caedf37cb0219d896be56
Merge "Minor fixes for the nova section (install guide)"