Update our handling of pbr.

*) We're removing our upper caps - they are just time bombs. Make pbr
   be consistent here too.

*) pbr 1.8 handles comments in marker lines properly. We don't need to
   update the run-time minimum, but we do need a build-time minimum of
   1.8 to ensure OpenStack packages can build properly.

Change-Id: I9fe0c62e774c00c2e521e1e69c5f2f5d87865ec1

global-requirements.txt

@@ -109,7 +109,7 @@ Parsley
 passlib>=1.6
 Paste
 PasteDeploy>=1.5.0
-pbr>=1.6,<2.0
+pbr>=1.6
 pecan>=1.0.0
 pexpect>=3.1,!=3.3 # ISC License
 Pillow>=2.4.0 # MIT

openstack_requirements/cmds/update.py

@@ -62,7 +62,7 @@ except ImportError:
     pass
 
 setuptools.setup(
-    setup_requires=['pbr>=1.3'],
+    setup_requires=['pbr>=1.8'],
     pbr=True)
 """