Installing snapd can hide dpkg prompts and make it look like it's stuck

[Why]

The new upstream version has two changes only (https://github.com/CanonicalLtd/ubuntu-advantage-script/releases/tag/v9)
- fix for this bug plus unit tests for it
- test suite refactoring

Were it not for the second change, I would not be asking for a Feature Freeze Exception (UPDATE: see comment #9). This second change split one big test python script into multiple modules, one per feature. That makes it easier to write new tests.

That test refactoring alone incurred in this small change to the main script, to ease its unit testing:
-SERIES=$(lsb_release -cs)
+SERIES=${SERIES:-$(lsb_release -cs)}

And:

+KERNEL_VERSION=${KERNEL_VERSION:-$(uname -r)}
(...)
-KERNEL_VERSION=$(uname -r)

These are the changes in the main script:
https://github.com/CanonicalLtd/ubuntu-advantage-script/compare/v8...v9#diff-3614ddfe4ef6c3b7b4df10ed10ec3095

And the full diff between the two releases:
https://github.com/CanonicalLtd/ubuntu-advantage-script/compare/v8...v9

[Travis CI]

Upstream has travis setup to test each landing branch:
https://travis-ci.org/CanonicalLtd/ubuntu-advantage-script/

Those tests are using python 3.5, while artful has python 3.6. I have attached a test log run on artful and python 3.6.

[PPA with daily builds]
We also have a PPA setup with daily builds from a github mirror using a launchpad recipe: https://code.launchpad.net/~ahasenack/+recipe/ubuntu-advantage-script-daily

Here is the v9 artful build:
https://code.launchpad.net/~ahasenack/+archive/ubuntu/ubuntu-advantage-tools-devel/+build/13335242

[Bugfix test]

This package is meant for LTS releases only, so it doesn't do much in artful other than to say it's not supported there. But since we will want to SRU it to the LTS releases, attached is a log of the trusty upgrade and livepatch enabling, which is what this bug is about.

==========================================================================

From the upstream report at https://github.com/CanonicalLtd/ubuntu-advantage-script/issues/54:

I tried to enable livepatch on a trusty machine, but it got stuck:

ubuntu@61-b1:~$ sudo ubuntu-advantage enable-livepatch <redacted>
Installing missing dependency snapd...

Behind the scenes, what happened is that dpkg issued a prompt:

2228 pts/0 S+ 0:00 | \_ sudo ubuntu-advantage enable-livepatch redacted
2229 pts/0 S+ 0:00 | \_ /bin/sh -e /usr/bin/ubuntu-advantage enable-livepatch redacted
2236 pts/0 S+ 0:01 | \_ apt-get install -y snapd
3269 pts/0 S+ 0:00 | \_ /usr/bin/dpkg --status-fd 43 --configure liblzo2-2:amd64 libsystemd-journal0:amd64 linux-image-4.4.0-92-generic:amd64 linux-image-extra-4.4.0-92-generic:amd64 linux-image-generic-lts-xenial:amd64 linux-headers-4.4.0-92:all linux-headers-4.4.0-92-generic:amd64 linux-headers-generic-lts-xenial:amd64 linux-generic-lts-xenial:amd64 cgroup-lite:all squashfs-tools:amd64 systemd:amd64 snapd:amd64 thermald:amd64
3278 pts/0 S+ 0:00 | \_ /usr/bin/perl /var/lib/dpkg/info/linux-image-4.4.0-92-generic.postinst configure
3304 pts/0 S+ 0:00 | \_ run-parts --verbose --exit-on-error --arg=4.4.0-92-generic --arg=/boot/vmlinuz-4.4.0-92-generic /etc/kernel/postinst.d
11905 pts/0 S+ 0:00 | \_ /usr/bin/perl -w /usr/share/debconf/frontend /usr/sbin/update-grub-legacy-ec2
11911 pts/0 S+ 0:00 | \_ /bin/bash /usr/sbin/update-grub-legacy-ec2
12312 pts/0 S+ 0:00 | | \_ /bin/bash /usr/bin/ucf --debconf-ok --debconf-template grub/update_grub_changeprompt_threeway --three-way /tmp/filez4NBAU /var/run/grub/menu.lst
12362 pts/0 S+ 0:00 | \_ whiptail --backtitle Package configuration --title --output-fd 11 --nocancel --default-item keep the local version currently installed --menu A new version of /boot/grub/menu.lst is available, but the version installed currently has been locally modified. What would you like to do about menu.lst? 17 118 7 -- install the package maintainer's version keep the local version currently installed show the differences between the versions show a side-by-side difference between the versions show a 3-way difference between available versions do a 3-way merge between available versions (experimental) start a new shell to examine the situation
Hitting ENTER at the stuck screen accepted whatever was the default choice and moved on.

Note that this can happen in any Ubuntu release, not just trusty.