FlowAPI should allow specifying cert files

[greenape]

For https, flowapi container requires: /run/secrets/cert-flowkit.pem and /run/secrets/key-flowkit.pem

Ideally, one should be able to specify names for these files, because to update a docker secret one creates a new secret which will have a new name.

This should be via env vars passed to the container, and continue to default to the current names.

One can work around this with the extended secrets syntax by specifying the name of the file, but it would be convenient to be able to do it this way as well.