zerovm-cli: don't run tests inside LXC

Our tests now require many different versions of Python and these
versions are only installed in the host OS. We don't really need LXC
for running tests -- the encapsulation done by tox is sufficient.

So now we only produce packages inside the LXC environment.

This should fix the errors we see in https://github.com/zerovm/zerovm-cli/pull/33.