Make oslosphinx optional

Avoid cyclic dependencies between pbr and oslosphinx. So if oslosphinx is not
available, continue to be able to generate the documentation.

Change-Id: I4c1f8ea5cded268388dab29931055223f8999c8a

doc/source/conf.py

@@ -8,8 +8,14 @@ sys.path.insert(0, os.path.abspath('../..'))
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ['sphinx.ext.autodoc',
-              'oslosphinx']
+extensions = ['sphinx.ext.autodoc']
+# make oslosphinx optional to not increase the needed dependencies
+try:
+    import oslosphinx
+except ImportError:
+    pass
+else:
+    extensions.append('oslosphinx')
 
 # autodoc generation is a bit aggressive and a nuisance when doing heavy
 # text edit cycles.