Support version override with PBR prefix.
Support version override with PBR prefix.

For non-OpenStack consumption of PBR, OSLO_PACKAGING_ is a weird
version override env var.

Change-Id: I8e80bcc5bc56e7950ed5cf0d24657957570134cf

openstack-dev/pbr,citrix-openstack-build/pbr,redhat-openstack/pbr,openstack-dev/pbr,redhat-openstack/pbr,klmitch/pbr,varunarya10/pbr,JioCloud/pbr,JioCloud/pbr,klmitch/pbr,citrix-openstack-build/pbr,WiFast/pbr,citrix-openstack-build/pbr,WiFast/pbr,varunarya10/pbr,openstack-dev/pbr