travis: run tests from inside VM + support test execution outside of Travis CI

This is a big improvement, because we can now communicate with
the running VM, including executing processes inside the VM, like
running goss directly from inside the VM.

While at it also adjust travis/execute.sh to be able to run it
*locally* as well. We now don't have to necessarily push to Travis
CI, which also serves to be useful when developing new features.

Thanks: Christian Hofstaedtler <zeha@debian.org> for helping me with the serial console handling