api, client: don't use a pointer-slice for plugins

The backend and router don't use a pointer-slice (the server actually
doesn't use the `plugin.ListResponse` type and a straight slice);
778e5bfad3/daemon/server/router/plugin/backend.go (L20)
6baf274fa3/daemon/server/router/plugin/plugin_routes.go (L276-L280)

Align the type in the API to match, and update the type defined in the
client accordingly.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

api/types/plugin/plugin_responses.go

@@ -5,7 +5,7 @@ import (
 )
 
 // ListResponse contains the response for the Engine API
-type ListResponse []*Plugin
+type ListResponse []Plugin
 
 // Privilege describes a permission the user has to accept
 // upon installing a plugin.