I am writing subordinate charm to neutron-api and I want to trigger neutron-db-manager upgrade head on neutron whenever my subordinate charm install hook is complete. How to do that?
It depends whether you assume that you would only ever be related to neutron-api and thus only colocated on machines where neutron-api has installed neutron-db-manager, or whether you would want neutron-api to coordinate the upgrade.
If you are running on the machine directly, then you could presumably just issue the neutron commands yourself. If not, then you would need to work with the neutron-api charm to define a relation that enables you to send it requests for what you want done.