Revise psycopg2 execute_values approach

Revise psycopg2 execute_values approach

Revised the approach for the just added support for the psycopg2
"execute_values()" feature added in 1.3.7 for :ticket:`4623`.  The approach
relied upon a regular expression that would fail to match for a more
complex INSERT statement such as one which had subqueries involved.   The
new approach matches exactly the string that was rendered as the VALUES
clause.

Fixes: #4623
Change-Id: Icaae0f7b6bcf87a2cf5c6290a839c8429dd5fac3
(cherry picked from commit 0a7ca00e04f7c1cfdbb8bdfe7da5f62fc9b40930)