r/netboot/jenkins-agent: Configure NBD exports

The *netboot/jenkins-agent* Ansible role configures three NBD exports:

* A single, shared, read-only export containing the Jenkins agent root
  filesystem, as a SquashFS filesystem
* For each defined agent host, a writable data volume for Jenkins
  workspaces
* For each defined agent host, a writable data volume for Docker

Agent hosts must have some kind of unique value to identify their
persistent data volumes.  Raspberry Pi devices, for example, can use the
SoC serial number.

roles/netboot/jenkins-agent/tasks/main.yml

@@ -0,0 +1,7 @@
+- name: ensure jenkins-agent nbd export is configured
+  template:
+    src: jenkins-agent.ini.j2
+    dest: /etc/nbd-server/config.d/jenkins-agent.conf
+    mode: u=rw,go=r
+  tags:
+  - nbd-config

roles/netboot/jenkins-agent/templates/jenkins-agent.ini.j2

@@ -0,0 +1,14 @@
+[jenkinsagent-aarch64]
+exportname = /var/lib/nbd/jenkinsagent-aarch64.squashfs
+readonly = true
+
+{% for name in netboot_jenkins_agents %}
+[docker-{{ name }}]
+exportname = /var/lib/nbd/docker-{{ name }}.img
+filesize = 10737418240
+
+[jenkins-ba5bebc5]
+exportname = /var/lib/nbd/jenkins-{{ name }}.img
+filesize = 10737418240
+#force_tls = true
+{% endfor %}