Add a separate Buildbot host info template for EC2

Latent workers are not usually created on the instance type that will be used,
so memory and CPU info collected at AMI creation will likely be inaccurate.

contrib/ci-workers/README.md

@@ -26,8 +26,8 @@ installation for use as a Buildbot worker in Zcash's CI.
     [zcash-ci-worker-unix]
     some-name ansible_host=<INSTANCE_IP> ansible_ssh_user=<USERNAME>
 
-- Run `ansible-playbook -i inventory/hosts unix.yml`, passing in the worker's
-  Buildbot name and password.
+- Run `ansible-playbook -e buildbot_worker_host_template=templates/host.ec2.j2 -i inventory/hosts unix.yml`,
+  passing in the worker's Buildbot name and password.
   - After a successful run, the worker should be connected to dev-ci.z.cash and
     visible in its worker list.
 

contrib/ci-workers/templates/host.ec2.j2

@@ -0,0 +1 @@
+OS: {{ ansible_distribution }} {{ ansible_distribution_version }}

contrib/ci-workers/unix.yml

@@ -104,7 +104,7 @@
 
     - name: Set host details for Buildbot worker
       template:
-        src: templates/host.j2
+        src: "{{ buildbot_worker_host_template }}"
         dest: "~/{{ buildbot_worker_name }}/info/host"
       become_user: "{{ buildbot_worker_user }}"
 

contrib/ci-workers/vars/buildbot.yml

@@ -2,3 +2,4 @@
 buildbot_worker_user: zcbbworker
 buildbot_master_host: dev-ci.z.cash
 buildbot_master_port: 9899
+buildbot_worker_host_template: templates/host.j2