use a consistently named "name" field when we report error in username
increase coverage in chef_users and ensure all tests pass

* add new tests for additional validation rules
* remove admin-related tests
* make some helper functions for common object creation in tests
* ensure that the validation errors are what we think they are
  by specifying field name in error as part of #ej_invalid{}

Still room to improve, coverage at 67% from ~50.