Wrap up virtual environment after creation

[Epiphane]

This is a solution for tox-dev/tox#283 - detox does not completely wrap up the virtual environment after creating it, and keeps recreating the venv for every execution.

Native tox behavior for the skipsdist case is located at https://github.com/msabramo/tox/blob/master/tox/_cmdline.py#L444 - calling venv.finish() writes the necessary configuration so that this environment will be reused in future runs.

[Epiphane]

Extra change - the return code from running detox is different than when running python -m detox. When running as a module, the return code is always successful, so raise a SystemExit in order to persist any erorrs.

[obestwalter]

Hey @Epiphane thanks a lot. A moment we are busy getting the next tox release out, but I am sure we can get to this soon.

[rm-you]

This is relevant to me too -- will someone be able to get to this? It's incredibly annoying for me, as I am sometimes working on projects with very large dependency trees and not the fastest internet, and it can take a very long time to regenerate envs...

[rm-you]

For now I've cherry-picked both of these commits locally and installed that in my virtualenv. Thanks for the patches, I really hope someone can merge this soon. 👍