Depend on pbr>=2.0.0
Depend on pbr>=2.0.0

Several of shade's dependencies now require pbr>=2.0 however the cap
is preventing `pip install shade` from just working after install
completes. Update shade to also require pbr>=2.0.0.

Change-Id: I5aa5081c200d24f9f43253d409f7b80e52c8841f
Signed-off-by: Thanh Ha <09ea4d3a79c8bee41a16519f6a431f6bc0fd8d6f@linuxfoundation.org>

dtroyer/python-openstacksdk,stackforge/python-openstacksdk,openstack-infra/shade,openstack/python-openstacksdk,stackforge/python-openstacksdk,openstack-infra/shade,openstack/python-openstacksdk,dtroyer/python-openstacksdk