API now allows for search of usergroups

That's about it. Usergroups had to be filtered on the client side to look for an specific name, way too much hurdle. Consider the case when a proxy has to do this every time it wants to create a host on foreman for a certain usergroup (real case). It can be argued the client could keep a list of ids - names but I think this makes things easier for anyone that wants to use the API.

Docs need to be updated accordingly