Add an installation guide with virtual environment

It is a common scenario that operators wanted to install Zun with an older version of OpenStack Services (e.g. nova, neutron). However, the existing installation guide [1] is not helpful for them because it assumes a latest version of OpenStack installation.

If people want to install Zun with older version of OpenStack, they should install Zun services using virtualenv [2]. It is better to have another installation guide to document how to do that.

[1] https://docs.openstack.org/zun/latest/install/
[2] https://virtualenv.pypa.io/en/stable/