Fix slave_connection string for unit tests
Fix slave_connection string for unit tests

oslo.config is becoming more strict with type enforcement [1], so we
cannot use False as slave_connection string.

[1] https://review.openstack.org/328692

Partial-Bug: #1683732
Change-Id: I6c6182df3a54d68737f8bbc77f1a2890ffcb0da2

openstack/ironic-inspector,openstack/ironic-inspector