pybind: Add BUILD_DOC env var to rbd/setup.py

Bypass sanity check if building docs.
Issue: http://tracker.ceph.com/issues/16940

Signed-off-by: Anirudha Bose <ani07nov@gmail.com>

src/pybind/rbd/setup.py

@@ -121,7 +121,11 @@ def check_sanity():
         shutil.rmtree(tmp_dir)
 
 
-if not check_sanity():
+if 'BUILD_DOC' in os.environ.keys():
+    pass
+elif check_sanity():
+    pass
+else:
     sys.exit(1)
 
 cmdclass = {}