Kill unattended upgrades, to avoid apt-get failures

[luke-jr]

Ubuntu Bionic tries to upgrade the system at boot by default, which breaks gitian's usage of apt-get.

[devrandom]

shouldn't we just uninstall unattended-upgrades during the VM build process?

[luke-jr]

Uninstalling it still requires doing this first (since apt-get remove may fail due to a running unattended upgrade..)

[devrandom]

vmbuilder has the --removepkg flag, which executes without the VM actually starting.

[devrandom]

(invoked from make-base-vm)

[luke-jr]

Ah, well, the root of the problem that got me here is that vmbuilder doesn't work with bionic...