It may be appropriate to provide some information on how to use python virtualenv

If people blindly follow the instructions on https://docs.egi.eu/users/cloud-compute/openstack/ using python may be confusing, leading to doing system-wide install, and may not always work as expected.

As python3 natively supports virtualenvs with just a couple of commands it may be helpful to provide some short info like on how to setup and use a virtualenv to do things cleanly:

```shell
mkdir ~/.virtualenvs
# or python depending on the name of your python 3 binary
python3 -m venv ~/.virtualenvs/fedcloud
# This command will have to be run to enable the fedcloud virtualenv and access the fedcloud commands
source ~/.virtualenvs/fedcloud/bin/activate
```