blacklist blockdiag 2.0.0

blockdiag has been used as an indirect dependcy via some sphinx
extensions such as sphinxcontrib-blockdiag and sphinxcontrib-actdiag
for several releases. For the past few releases nova has monkey
patched blockdiag to work around there global replacement
of codec.getreader under python 3.

The 2.0.0 release broke how that monkey patching worked and also
was the first release in several years. Now that the project is active
again we have reported the python 3 issue to the maintainer
https://github.com/blockdiag/blockdiag/issues/126 and it
has been adressed by https://github.com/blockdiag/blockdiag/pull/129
in 2.0.1 which has now been released.
https://pypi.org/project/blockdiag/2.0.1/

This change blacklists 2.0.0. This change does not raise the minimum
requirement. Nova can selectively raise its minimum requirement in the
future in tree if it chooses to remove support for monkey patching
older releases.

Other project are recommended to use 2.0.1 or later as this is the first
release which is python3 only and is compatible with the sphinx
extentions we use without monkey patching.

Change-Id: I414d9e604616d49b4456ebc348418e9b1b09058b

global-requirements.txt

@@ -12,6 +12,7 @@ Babel!=2.4.0  # BSD
 bcrypt  # Apache-2.0
 betamax  # Apache-2.0
 betamax-matchers  # Apache-2.0
+blockdiag!=2.0.0  # Apache-2.0
 boto  # MIT
 boto3  # Apache-2.0
 botocore  # Apache-2.0