Fix TangoAuthority is called twice for each attribute

Fixes #1298 (closed) The method getAuthority was being called from the getDevice method without the schema on the name. This caused that the Factory identified it as a different database, creating a new Authority object. By adding fixing this we can see a massive difference in taurus core startup time. This improvement is scalable, it saves time for each attribute read. These examples show the time before and after the fix, one example with 400 attributes and another one with 4000.