CI: fix ceph-ansible installation after cryptography 3.4 release

Installing ceph-ansible in the virtualenv on CentOS 8 fails with:

    ModuleNotFoundError: No module named 'setuptools_rust'

This error appeared following the release of cryptography 3.4, which now
includes Rust code. It can be installed without Rust using a Python
wheel, but only with more recent pip than version 9.0.3 available as RPM
on CentOS 8. The cryptography bug report [1] recommends pip>=19.1.1.

This change upgrades pip in the virtualenv before installing
ceph-ansible.

[1] https://github.com/pyca/cryptography/issues/5753

Change-Id: I47473de6f71c422db2238d653c2d8f379c55e79b