KEYCLOAK-621 fix searchForUserByAttributes method

* JpaUserProvider.searchForUserByAttributes now works (it always returned an empty List before this)
* In addition, always use query parameters, so that there is no possibility of SQL injection in this method
* I had to use new variables for the parameter names (e.g. EMAIL, USERNAME, etc.) because the equivalents in UserModel had brackets, which are not allowed in parameter names.
* I did not write any automated tests, so this can't really be considered done.