Add function drop_unique_constraint()

We forgot to copy this function from nova, so we are not able to
switch to oslo sqlalchemy.utils until add this function.

Sqlalchemy doesn't supports some sqlite column types so we have no
possibility to drop unique constraint in general way on sqlite (we
loose these column types after migration).
Added unified drop_unique_constraint() function to drop unique
constraint for sql backends with tests.

blueprint: oslo-sqlalchemy-utils

Change-Id: I34000ad2277a97c31a29539d047faaf19876c9d5
Merge "Add function drop_unique_constraint()"