Use master branch of openstacksdk in Bifrost jobs

Bifrost downgrades openstacksdk to a release <0.99.0 [1], which is
incompatible to our master branch of the Ansible OpenStack
collection. By adding openstacksdk to Zuul variable job.required-\
projects [2], Bifrost will install openstacksdk from its master
branch, overwriting any existing (older) openstacksdk releases.

[1] https://opendev.org/openstack/bifrost/commit/03ddd026567f7b61b89d83b0e7e7cb35ed466ae6
[2] https://review.opendev.org/c/openstack/bifrost/+/849563/5/playbooks/test-bifrost.yaml

Change-Id: I46dadc70338bb45d459e8176cc26e67c20c1d4ae