Revert "Switch to Ubuntu 16.04."
Revert "Switch to Ubuntu 16.04."

It turned out to be a) too bloaty (cloud init and other auto-magical stuff)
b) too unstable (random freezes and/or sshd failures after or during booting)

This reverts commit 704980516fc9f72a09e19d1c817a2a775869a145.

grunskis/nuimo-hub-backend,grunskis/senic-hub,grunskis/nuimo-hub-backend,getsenic/senic-hub,grunskis/senic-hub,grunskis/senic-hub,getsenic/senic-hub,grunskis/senic-hub,grunskis/nuimo-hub-backend,grunskis/nuimo-hub-backend,grunskis/senic-hub,grunskis/nuimo-hub-backend,grunskis/senic-hub