packages: Add framework for installing packages

Some machines may need to install multiple packages for separate use
cases.  Requiring each use case to define a systemd unit that runs
`rpm-ostree install` directly would be cumbersome and also quite slow,
as each one would have to run in turn.  Instead, now there is a single
*install-packages.service* which installs all of the packages listed in
files in `/etc/ignition/packages.d`.  On first boot, all files in that
directory are read and all the packages they list will be installed in a
single `rpm-ostree install` invocation.

install-packages.service

@@ -0,0 +1,17 @@
+# vim: set ft=systemd :
+[Unit]
+Description=Install collectd
+After=network-online.target
+Wants=network-online.target
+Before=zincati.service
+ConditionPathExists=/etc/ignition/packages.d
+ConditionPathExists=/etc/ignition/packages.installed
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/bin/sh /etc/ignition/install-packages.sh
+ExecStartPost=/bin/touch /etc/ignition/packages.installed
+
+[Install]
+WantedBy=multi-user.target

install-packages.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+if [ ! -d /etc/ignition/packages.d ]; then
+    exit 0
+fi
+
+cat /etc/ignition/packages.d/* | xargs rpm-ostree install --apply-live -y

packages.yaml

@@ -0,0 +1,16 @@
+variant: fcos
+version: 1.4.0
+
+storage:
+  directories:
+  - path: /etc/ignition/packages.d
+    mode: 0755
+  files:
+  - path: /etc/ignition/install-packages.sh
+    mode: 0755
+    contents:
+      local: install-packages.sh
+  - path: /etc/systemd/system/install-packages.service
+    mode: 0644
+    contents:
+      local: install-packages.service