The 81b01a191a46_pjsip_add_contact_reg_server.py script was attempting
to use UniqueConstraint and failing. It was not imported and after
importing it also continued to fail.

I've changed the script to use the explicit name of the constraint
instead.

Change-Id: I2438b0be90b7ce583b47dd27983c0c1a02cea5b9