Support adding pip packages

roles/linux_ansible_init/README.md

@@ -6,6 +6,10 @@ The ansible-init system consists of a script that is installed in the image and
 
 ansible-init is able to execute playbooks that are baked into the images, and also from arbitrary Ansible collections, e.g. from Galaxy or from a Git repository. These extra playbooks are specified by the Azimuth operator in their Azimuth configuration, and communicated to the instances that make up platforms using cloud-init instance metadata. This allows site- or platform-specific customisations to be applied to instances in a consistent way without modifying the images.
 
+## Role variables
+
+- `ansible_init_pip_packages`: Optional. List of packages to install into venv.
+
 ## Usage
 
 The 255 byte limit per key/value pair for instance metadata means that collections will be defined by metadata items of the following structure:

roles/linux_ansible_init/defaults/main.yml

@@ -0,0 +1,4 @@
+ansible_init_pip_packages:
+  - ansible
+  - jmespath
+  - requests

roles/linux_ansible_init/tasks/main.yml

@@ -17,10 +17,7 @@
 
 - name: Install dependencies into virtualenv
   pip:
-    name:
-      - ansible
-      - jmespath
-      - requests
+    name: "{{ ansible_init_pip_packages }}"
     state: latest
     virtualenv: /usr/lib/ansible-init
     virtualenv_command: "{{ python3_cmds[ansible_facts['distribution']] }} -m venv --upgrade-deps"