[RFE] introduce explicit database constraint naming conventions

Based on the discussion in the multi-attribute node look-up patch[1]

We didn't explicitly name database constraints before.
This leads to issues if we want to modify/drop constraints in a future schema update[2]

[1] https://review.openstack.org/#/c/421285/7/ironic_inspector/migrations/versions/882b2d84cb1b_attribute_constraints_relaxing.py@32
[2] http://docs.sqlalchemy.org/en/latest/core/constraints.html#configuring-constraint-naming-conventions