Enforce sqlite-specific flow in drop_unique_constraint

The nova.db.sqlalchemy.apy.drop_unique_constraint method has a
sqlite-specific flow for dropping unique constraints because it can't
use alter table like mysql or postgresql. The code is explicitly
checking if the db engine is mysql or postgresql and then falls into a
block which assumes the backing db is sqlite. This can cause problems if
the backing db is not sqlite.

This patch simply switches the checks so that the sqlite-specific flow
is only ran if the backing db is sqlite, everything else will default to
the 'alter table' flow.

Change-Id: I23b9bf33d18df3ac36106d5800a8ecdd1a4018df
Merge "Enforce sqlite-specific flow in drop_unique_constraint"