Simplify inserting apt keys into LXC image

Bionic requires a functioning gpg-agent to run apt-key add. This means
that gpg-agent must be working properly in the chroot when the lxc image
preparation script runs.

Previous changes [1] have enabled apt-key to communicate with gpg-agent
during the lxc_hosts role checks, however the cache prep fails almost
every time when lxc_hosts is used within other role checks.

This is not a new issue, debian-installer is affected too [2].

This change adopts the same route as d-i, and simply copies the host
/etc/apt/trusted.gpg.d directory to the LXC image, removing the
need for apt-key and in turn gpg-agent.

[1] https://review.openstack.org/#/c/588629/
[2] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=851774

Change-Id: Ia511881f675cd88ecc58360b18531f0dac486b70

templates/prep-scripts/ubuntu_18_prep.sh.j2

@@ -14,12 +14,6 @@ apt-get remove -y --purge snap* lxc* lxd* resolvconf* || true
 apt-get update
 apt-get install -y -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" --force-yes gnupg
 
-#start gpg-agent if is is not already running
-gpg-connect-agent /bye
-
-apt-key add -v /root/repo.keys
-rm /root/repo.keys
-
 apt-get upgrade -y
 apt-get install -y -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" --force-yes {{ lxc_cache_distro_packages | join(' ') }}
 apt-get upgrade -y -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" --force-yes

vars/ubuntu-18.04.yml

@@ -22,10 +22,10 @@ _lxc_cache_map:
   copy_from_host:
     - /etc/apt/sources.list
     - /etc/apt/apt.conf.d/
+    - /etc/apt/trusted.gpg.d
     - /etc/apt/preferences.d/
     - /etc/environment
     - /etc/localtime
-    - /root/repo.keys
     - /etc/protocols
 
 _lxc_cache_prep_template: "prep-scripts/ubuntu_18_prep.sh.j2"