Use latest pip in gate jobs

We've seen failures with probe tests lately where dnspython 2.0.0 is
getting installed even though it doesn't support py2 anymore. I think
using latest pip should be better about noticing that and installing the
last 1.x release intead?

Change-Id: I6eda54ccd2792effadb334ce9324887132b62b6f

tools/playbooks/common/install_dependencies.yaml

@@ -17,6 +17,11 @@
   roles:
     - ensure-pip
   tasks:
+    - name: upgrade pip
+      pip:
+        name: pip
+        extra_args: --upgrade
+
     - name: installing dependencies
       yum: name={{ item }} state=present
       with_items: