Mitigate 'no "man_pages" config value' error

For some reason, the gate was failing with the following message:

    "WARNING: no "man_pages" config value found; no manual pages will be
    written"

I don't know why this has suddenly started happening, but it was
mitigated by 'I97d4ade4'. However, this change didn't fix the root cause
of the issue, which is that 'pbr' defaults to building both html and man
page output and we don't need/want the latter.

Disable the man page builder entirely. This allows us to remove all man
page-related configuration in the process, seeing as it'll never be
used.

Change-Id: I83bb54f413cbad29afb2233043150fb037bca3bd

doc/source/conf.py

@@ -93,9 +93,6 @@ pygments_style = 'sphinx'
 # A list of ignored prefixes for module index sorting.
 modindex_common_prefix = ['nova-specs.']
 
-# -- Options for man page output ----------------------------------------------
-#man_pages = []
-
 # -- Options for HTML output ---------------------------------------------------
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for

requirements.txt

@@ -1,8 +1,8 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
+
 oslosphinx>=4.7.0 # Apache-2.0
-pbr>=1.8 # Apache-2.0
 sphinx>=1.5.1 # BSD
 testrepository>=0.0.18 # Apache-2.0/BSD
 testtools>=1.4.0 # MIT

setup.cfg

@@ -12,6 +12,7 @@ classifier =
     Operating System :: POSIX :: Linux
 
 [build_sphinx]
+builders = html
 all_files = 1
 build-dir = doc/build
 source-dir = doc/source

setup.py

@@ -25,5 +25,5 @@ except ImportError:
     pass
 
 setuptools.setup(
-    setup_requires=['pbr>=1.8'],
+    setup_requires=['pbr>=2.0'],
     pbr=True)