Add note about neutron-db-manage

I added a note to cover occasional instances where users
may need to run neutron-db-manage to populate the neutron
database. I want to leave this note out of the Juno version
for now.

Change-Id: Ia80f8f8f3768467c59dab98e0e5569aeb211206e
Closes-Bug: #1343277
Merge "Add note about neutron-db-manage" into stable/icehouse