Fix multiple CI failures

1) Bump hacking max version to 3.0.1 and fix code style failures.

pep8 job is failing because the version of flake8 (flake8<2.7.0,>=2.6.0)
requested by hacking<1.2.0 is not compatible with pyflakes>=2.1.1.
Therefore the patch increases the max version of hacking.

2) Disable swift-proxy service which fails to start on stein. Disabled
   other unneeded services while here.

Change-Id: I87c6d037fbbc24ef795ddbe26b5e297fb1bbc7a9

.zuul.yaml

@@ -85,5 +85,18 @@
     nodeset: openstack-single-node-bionic
     override-checkout: stable/stein
     vars:
+      devstack_services:
+        # Disable unneeded services.
+        etcd3: false
+        s-account: false
+        s-container: false
+        s-object: false
+        s-proxy: false
+        c-api: false
+        c-bak: false
+        c-sch: false
+        c-vol: false
+        cinder: false
+        horizon: false
       devstack_localrc:
         USE_PYTHON3: True