Update unattended-upgrades config to avoid conffile prompts. (#781)

This commit updates the `50unattended-upgrades` configuration file that
Streisand places in `/etc/apt/apt.conf.d` to add additional options for
how to handle dpkg conffile conflicts.

If a package (e.g. `tor`) has a security update and a change to a config
file (e.g. `/etc/tor/torrc`) then the update will stall at a dpkg
conffile prompt asking if the user wants to keep the modified Streisand
config file or the new package one. We want to leave the Streisand
configuration in place 100% of the time and have the new config file
placed next to it with a `.dpkg-dist` suffix.

To do so the `--force-confdef` and `--force-confold` dpkg options are
used. More about these options and the dpkg conffile process can be
found here:
https://raphaelhertzog.com/2010/09/21/debian-conffile-configuration-file-managed-by-dpkg/

playbooks/roles/common/files/50unattended-upgrades

@@ -30,3 +30,10 @@ Unattended-Upgrade::Automatic-Reboot "true";
 // time instead of immediately
 //  Default: "now"
 Unattended-Upgrade::Automatic-Reboot-Time "00:00";
+
+// Avoid conffile dpkg prompt by *always* leaving the modified configuration in
+// place and putting the new package configuration in a .dpkg-dist file
+Dpkg::Options {
+   "--force-confdef";
+   "--force-confold";
+};