[AIRFLOW-3089] Drop hard-coded url scheme in google auth redirect. (apache#3919)

The google auth provider hard-codes the `_scheme` in the callback url to
`https` so that airflow generates correct urls when run behind a proxy
that terminates tls. But this means that google auth can't be used when
running without https--for example, during local development. Also,
hard-coding `_scheme` isn't the correct solution to the problem of
running behind a proxy. Instead, the proxy should be configured to set
the `X-Forwarded-Proto` header to `https`; Flask interprets this header
and generates the appropriate callback url without hard-coding the
scheme.

Author: jmcarp

airflow/contrib/auth/backends/google_auth.py

@@ -112,8 +112,7 @@ def login(self, request):
         log.debug('Redirecting user to Google login')
         return self.google_oauth.authorize(callback=url_for(
             'google_oauth_callback',
-            _external=True,
-            _scheme='https'),
+            _external=True),
             state=request.args.get('next') or request.referrer or None)
 
     def get_google_user_profile_info(self, google_token):