Bug 14256: Check for unique constraint to regenerate random data

Bug 14256: Check for unique constraint to regenerate random data

Unique constraints should be checked when creating random data. Otherwise
we get failures when the generated data already exists on the DB.

This patch takes advantage of ->unique_constraints() to do the job,
looping through all the unique constraints defined for the source.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Mason James <mtj@kohaaloha.com>