Bug 14256: (follow-up) Check for unique constraint to regenerate random data

Bug 14256: (follow-up) Check for unique constraint to regenerate random data

There were some issues in the previous patch. This patch fixes the
following:
- rename $value with $original_value
- remove $at_least_one_constraint_failed and $values_ok which make the
  code unnecessarily complicated
- the constraints have to be checked only if no original value is passed
- _buildColumnValue created a key to the default value hashref, it broke
  the test:
    last BUILD_VALUE if exists( $default_value->{$source} );

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>