[ActsAsMiqSet] Have add_member track new members
[ActsAsMiqSet] Have add_member track new members

The previous implementation of `.add_member`, the `RelationshipMixin`
would keep track of the newly added members, and the new implementation
did not.

This was used once or twice in a controller validation, so putting the
return style back in place to keep the same interface and fix tests.

NickLaMuro/manageiq,ManageIQ/manageiq,agrare/manageiq,jrafanie/manageiq,ManageIQ/manageiq,chessbyte/manageiq,mzazrivec/manageiq,agrare/manageiq,chessbyte/manageiq,chessbyte/manageiq,kbrock/manageiq,mzazrivec/manageiq,agrare/manageiq,kbrock/manageiq,mzazrivec/manageiq,agrare/manageiq,kbrock/manageiq,jrafanie/manageiq,jrafanie/manageiq,chessbyte/manageiq,NickLaMuro/manageiq,NickLaMuro/manageiq,NickLaMuro/manageiq,kbrock/manageiq,mzazrivec/manageiq,ManageIQ/manageiq,ManageIQ/manageiq,jrafanie/manageiq