ci: Factor out image prep bootstrap as a role

Promoting the script to a full template will fix some whitespace errors later.

tests/image_prep/_container_setup.yml

@@ -2,18 +2,8 @@
   hosts: all
   strategy: linear
   gather_facts: false
-  tasks:
-    - name: Install bootstrap packages
-      raw: |
-        set -o errexit
-        set -o nounset
-        if type -p yum; then
-          yum -y install {{ bootstrap_packages | join(' ') }}
-        else
-          apt-get -y update
-          apt-get -y --no-install-recommends install {{ bootstrap_packages | join(' ') }}
-        fi
-      when: bootstrap_packages | length
+  roles:
+    - role: bootstrap
 
 - name: Setup containers
   hosts: all

tests/image_prep/roles/bootstrap/defaults/main.yml

@@ -0,0 +1 @@
+bootstrap_packages: []

tests/image_prep/roles/bootstrap/tasks/main.yml

@@ -0,0 +1,3 @@
+- name: Bootstrap
+  raw: "{{ lookup('template', 'bootstrap.sh.j2') }}"
+  changed_when: true

tests/image_prep/roles/bootstrap/templates/bootstrap.sh.j2

@@ -0,0 +1,13 @@
+set -o errexit
+set -o nounset
+
+{% if bootstrap_packages %}
+if command -v apt-get; then
+  apt-get -y update
+  apt-get -y --no-install-recommends install {{ bootstrap_packages | join(' ') }}
+elif command -v yum; then
+  yum -y install {{ bootstrap_packages | join(' ') }}
+else
+  exit 42
+fi
+{% endif %}