Move api_class options to where they are used

The compute_api_class, network_api_class and volume_api_class options
are only used in their respective modules, so move them there.

blueprint: scope-config-opts
Change-Id: Ib0497f46b44c4b3af7319550cc64c09d629ea362
Move osapi_compute_unique_server_name_scope to db

This strange little option is only used within the DB layer.

blueprint: scope-config-opts
Change-Id: I10b0fef02a5fffd44094fe351cde683fc5c31442