The neutron-server service behavior in Juno has changed a bit since Icehouse.
Prior to Juno, when one went to start up neutron-server for the first time, an initial db sync was performed which generates the database schema.
One who attempts to intall Neutron on Juno without running a neutron-db-manage will not be able to start neutron-server and see the following error or similar in /var/log/neutron/server.log:
ProgrammingError: (ProgrammingError) (1146, "Table 'neutron.ml2_gre_allocations' doesn't exist") 'SELECT ml2_gre_allocations.gre_id AS ml2_gre_allocations_gre_id, ml2_gre_allocations.allocated AS ml2_gre_allocations_allocated \nFROM ml2_gre_allocations' ()
To fix the issue, run the following prior to starting neutron-server:
sudo neutron-db-manage --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugins/ml2/ml2_conf.ini upgrade juno