Match the minimum from global-requirements in setup.py

It's somewhat strange to say you can build a package with pbr >= 1.8 but
to install it you need 2.0.0.  Synchronise these values.

Change-Id: I47640a0226be2e82f2fbbf866d2c235f5e7ed415

openstack_requirements/cmds/update.py

@@ -63,7 +63,7 @@
     pass
 
 setuptools.setup(
-    setup_requires=['pbr>=1.8'],
+    setup_requires=['pbr>=2.0.0'],
     pbr=True)
 """