Adjust auth_get_auth_instances_for_wwwroot() to handle the new institution.id column

Bug 1323911: Now that the auth_instance table and the institution table both have a
column called "id", the SQL query is ambiguous about which one it will return.
Cutting the query down so that it only returns the fields actually used in the code
resolves this ambiguity.

Change-Id: I5242aa7bdee38af9aa0a7da308bb0d30dc700621
Signed-off-by: Yuliya Bozhko <yuliya.bozhko@totaralms.com>