Custom virtual environment
This MR follows !1135 (merged).
Currently, slapos-testing and slapos-sr-testing create a virtual testing environment.
Their virtual environments work almost like virtual-env.cfg, so I modified my macro to create a fully customized virtual environment.
I added the environment option which allows you to choose your custom environment variables.
I also added a message option that prints a string.
Finally, I modified slapos-testing and slapos-sr-testing to use my virtual-env macro instead of a hard-coded virtual environment.
The interest is that now we can know if you are in a test environment.
Moreover, you can deactivate the environment.