curtainer: does not work with xenial minimal images

https://github.com/canonical/curtin/commit/820f4603b12fa1b5649d38568d8feb80091a410d introduced support for Ubuntu minimal images, which don't include add-apt-repository. Unfortunately, it assumes that ASCII-armored keys can be places in /etc/apt/trusted.gpg.d/ across all Ubuntu releases: this is only true as of apt 1.4 (https://manpages.debian.org/buster/apt/apt-key.8.en.html#SUPPORTED_KEYRING_FILES) and xenial has apt 1.2.x.

The relevant log lines from a failing build:

00:17:40 enabling daily: ppa:curtin-dev/daily
00:17:40 lxc exec --mode=non-interactive curtin-vmtest-daily-x-627 -- which add-apt-repository </dev/null
00:17:40 lxc exec --mode=non-interactive curtin-vmtest-daily-x-627 -- sh -c
00:17:40 echo deb http://ppa.launchpad.net/curtin-dev/daily/ubuntu xenial main > /etc/apt/sources.list.d/curtin-daily-ppa.list &&
00:17:40 wget -q "https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x1bc30f715a3b861247a81a5e55fe7c8c0165013e" -O /etc/apt/trusted.gpg.d/curtin-daily-ppa.asc </dev/null
00:17:40 lxc exec --mode=non-interactive curtin-vmtest-daily-x-627 -- sh -c
00:17:40 echo deb-src http://ppa.launchpad.net/curtin-dev/daily/ubuntu xenial main >> /etc/apt/sources.list.d/curtin-daily-ppa.list </dev/null
00:17:40 lxc exec --mode=non-interactive curtin-vmtest-daily-x-627 -- sh -c
00:17:40 rm -f /var/lib/apt/lists/*Translation*;
00:17:40 echo "$1" > "$2" -- Acquire::Languages "none"; /etc/apt/apt.conf.d/99notranslations </dev/null
00:17:40 lxc exec --mode=non-interactive curtin-vmtest-daily-x-627 -- eatmydata apt-get -q update </dev/null
00:17:41 Get:1 http://security.ubuntu.com/ubuntu xenial-security InRelease [109 kB]
00:17:41 Hit:2 http://archive.ubuntu.com/ubuntu xenial InRelease
00:17:41 Get:3 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [109 kB]
00:17:41 Get:4 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages [1456 kB]
00:17:41 Get:5 http://security.ubuntu.com/ubuntu xenial-security/restricted amd64 Packages [9824 B]
00:17:41 Get:6 http://security.ubuntu.com/ubuntu xenial-security/universe amd64 Packages [754 kB]
00:17:41 Get:7 http://security.ubuntu.com/ubuntu xenial-security/multiverse amd64 Packages [8228 B]
00:17:41 Get:8 http://ppa.launchpad.net/curtin-dev/daily/ubuntu xenial InRelease [18.1 kB]
00:17:41 Ign:8 http://ppa.launchpad.net/curtin-dev/daily/ubuntu xenial InRelease
00:17:41 Get:9 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [107 kB]
00:17:41 Get:10 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages [7532 kB]
00:17:41 Get:11 http://ppa.launchpad.net/curtin-dev/daily/ubuntu xenial/main Sources [676 B]
00:17:41 Get:12 http://ppa.launchpad.net/curtin-dev/daily/ubuntu xenial/main amd64 Packages [1048 B]
00:17:42 Get:13 http://archive.ubuntu.com/ubuntu xenial/multiverse amd64 Packages [144 kB]
00:17:42 Get:14 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages [1861 kB]
00:17:42 Get:15 http://archive.ubuntu.com/ubuntu xenial-updates/restricted amd64 Packages [10.2 kB]
00:17:42 Get:16 http://archive.ubuntu.com/ubuntu xenial-updates/universe amd64 Packages [1178 kB]
00:17:42 Get:17 http://archive.ubuntu.com/ubuntu xenial-updates/multiverse amd64 Packages [23.0 kB]
00:17:42 Get:18 http://archive.ubuntu.com/ubuntu xenial-backports/main amd64 Packages [9812 B]
00:17:42 Get:19 http://archive.ubuntu.com/ubuntu xenial-backports/universe amd64 Packages [11.3 kB]
00:17:46 Fetched 13.3 MB in 3s (4271 kB/s)
00:17:46 Reading package lists...
00:17:47 W: GPG error: http://ppa.launchpad.net/curtin-dev/daily/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 55FE7C8C0165013E
00:17:47 W: The repository 'http://ppa.launchpad.net/curtin-dev/daily/ubuntu xenial InRelease' is not signed.