Fix sqlalchemy utils.

This patch affects drop_unique_constraint() on sqlite engine.
1. Fix error for tables with foreign key(s).
2. Now work with tables contains reserved words in
column names ("key" for example).

Fix bug 1195216.

Change-Id: Idc4dfa5006eee115f578b00d675cae1d6241ec69
Merge "Fix sqlalchemy utils."