Update neutron content for Juno

I updated the neutron content in the installation guide for Juno
as follows:

1) Reinstated the 'neutron-db-manage' database population step
   because the RC2 packages required it.
2) Explicitly removed 'connection' options on network and compute
   nodes because the default configuration on some distributions
   reference a SQLite database that could cause problems.

Change-Id: I663b4effbc52071de2783d3c4018cc970b5fb14b
Merge "Update neutron content for Juno"