Ensure namespace package is installed

This is apparently fixed in pbr since I3972b3132619e8e2dd7e362ca5fe9d1e3add43b8
but I'm seeing the issue with pbr 0.5.21 on Fedora.

Related-Bug: #1194742
Change-Id: I136b8493c8d8d48a0116facf5f23c2a1479c070f