Use schema_name() for SQL Server default schema

Use schema_name() for SQL Server default schema

Changed the query used to get "default schema name", from one that
queries the database principals table to using the
"schema_name()" function, as issues have been reported that the
former system was unavailable on the Azure Data Warehouse edition.
It is hoped that this will finally work across all SQL Server
versions and authentication styles.

Change-Id: Ic11bd4162c0d6a60432ae44876e86512703c1f81
Fixes: #3810
(cherry picked from commit 562e37ee7eaab8b19a9f081d4b170b95fce4f619)