Server opa should be able to Listen to a Unix Domain Socket

[FrankJLhota]

This would be handy for testing, and for instances where opa services multiple local processes.

[tsandall]

This question has come up from a few users and while it's not been a blocker, it would be nice to have.

From an implementation point of view, the following changes should be made (minimally):

• Add an extra listener loop in server.go. The listener should open/create a unix domain socket and then serve the Server on it.

• Plumb a file path argument for the domain socket through from the cmd package to the server package. See other server parameters for examples.

• If feasible, add an integration test that starts the server with unix domain socket listener enabled and validates that the HTTP API is served correctly.

[JAORMX]

Addressing this here #752

[JAORMX]

Fixed in #752

Should this be closed?

[tsandall]

Done.