Don't run main() on import

This patch adds the usual __name__ test to the database migration script to
avoid execution of the main() function in case of a module import (which
happens when running the configuration options discovery script from
openstack-doc-tools).

Closes-Bug: #1290708
Change-Id: I863a36bcbbbbb02652a4aae8ec1f383c43b5bc56
Merge "Don't run main() on import"