ci: Handle custom package repositories in bootstrap role

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

@@ -1 +1,2 @@
 bootstrap_packages: []
+package_manager_repos: []

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

@@ -1,6 +1,12 @@
 set -o errexit
 set -o nounset
 
+{% for item in package_manager_repos %}
+cat << "EOF" > "{{ item.dest }}"
+{{ item.content }}
+EOF
+{% endfor %}
+
 {% if bootstrap_packages %}
 if command -v apt-get; then
   apt-get -y update