The requirements.txt contents do not need to be
installed on to the host. The majority of the
requirements are for ansible, or for release
and management tooling which needs to use the
Ansible runtime venv.
Rather than forcing the installation of pip on
the host, we only install virtualenv via distro
packages (where possible). With virtualenv in
place we can create the runtime venv and install
pip, etc and all requirements into there.
Doing this keeps the system python libraries as
clean as possible, preventing clashes with other
packages (eg: ceph) which try to install other
python libraries which conflict on CentOS.
Change-Id: I0db786645c11649764680697518c97ddf9610cfa
b95eafb0ee