Updates to the neutron install section

Update a few elements to try to make things clearer:
- turn the warning about commenting [service_providers] into a step to
  make sure that the user will do it.
- use a keystone command to compute the SERVICE_TENANT_ID value
  (RHEL/SUSE).
- simplify the 'ln -s' command to avoid changing directory.

Change-Id: Id76ba3a4ade0753522a4d49bdd4d8bcec8b56d62
Merge "Updates to the neutron install section"