Description of problem:
The API does not have a possiblity to list all roles assigned to a user

Currently the API provides only a GET request to list all users for a role:

/usr/share/foreman/app/controllers/api/v2# grep roles *
roles_controller.rb:      api :GET, "/roles/", N_("List all roles")
roles_controller.rb:        @roles = resource_scope_for_index
roles_controller.rb:      api :GET, "/roles/:id/", N_("Show a role")
roles_controller.rb:      api :POST, "/roles/", N_("Create a role")
roles_controller.rb:      api :PUT, "/roles/:id/", N_("Update a role")
roles_controller.rb:      api :DELETE, "/roles/:id/", N_("Delete a role")
users_controller.rb:      api :GET, "/roles/:role_id/users", N_("List all users for role")

There is no route for GET /users/:user_id/roles


Version-Release number of selected component (if applicable):


How reproducible:


Steps to Reproduce:
1. Use the API to list roles assigned to a user
2.
3.

Actual results:
No API call available

Expected results:
GET /users/:user_id/roles available

Additional info: