Install swift keystone extras requirements

Since the introduction of I8f24b839bf42e2fb9803dc7df3a30ae20cf264
s-proxy is no longer able to launch as keystonemiddleware (listed under
test-requirements.txt) has not been installed.

keystonemiddleware is listed as extras requirements in swift
- https://github.com/openstack/swift/blob/e0d46d77fa740768f1dd5b989a63be85ff1fec20/setup.cfg#L79

Let's install swift keystone extra requirements also.

Closes-Bug: #1909018
Change-Id: I02c692e95d70017eea03d82d75ae6c5e87bde8b1
(cherry picked from commit 04b0b615)