setup up virtualenv if not exists

[fgregg]

the current deploy script assumes that an appropriate virtualenv already exists. The script should not make this assumption.

[reginafcompton]

@fgregg We have a step for this in the deploy-a-site documentation: https://github.com/datamade/deploy-a-site/blob/master/Basic-server-setup.md#make-a-virtual-environment

It's unlikely that someone would create deploy scripts before making a virtualenv (step 2 of the deploy a Python app).

If you want us to add an explicit "mkvirtualenv" in the before_install script (or somewhere along those lines), that's possible...but could you open that issue in the the datamade/devops repo?