certbot-dns-google: enable automatic credential lookup on google cloud (#5117)

- when no credentials are passed it will try to get valid credentials
using the google metadata service
- this is a feature of the google SDK, so we don't need to handle that
explicitly
- previous behaviour with a credentials file is retained

certbot-dns-google/setup.py

@@ -19,6 +19,8 @@ install_requires = [
     # will tolerate; see #2599:
     'setuptools>=1.0',
     'zope.interface',
+    # already a dependency of google-api-python-client, but added for consistency
+    'httplib2'
 ]
 
 docs_extras = [