Use latest pip in gate jobs
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

openstack/swift,swiftstack/swift,tipabu/swift,tipabu/swift,openstack/swift,swiftstack/swift,tipabu/swift,openstack/swift,tipabu/swift,swiftstack/swift,openstack/swift