Add testrepository build dependency to taskflow
Add testrepository build dependency to taskflow

oslotest has dropped its runtime dependency on testrepository[1] but
this package still relies on it, so add it as a build dependency until
it switches to stestr.

[1] https://review.openstack.org/569914

Change-Id: I49d5c16307daeaa87995dbd17306a16fa93aa8f2

openstack/rpm-packaging,openstack/rpm-packaging