fixes #5780 - API v2 - RESTful way to add/remove many-to-many association one at a time rather than passing all id's

Note: Related to PR https://github.com/theforeman/foreman/pull/1773 to add auditing when adding/removing
The parameters are marked required which is actually not true. Only one of them at a time is required. Apipie does not have means to express it so I vote for making them optional.
The same applies also to the other controllers.