Set become method to su

Since the host-provisioner connects to remote machines as _root_, it
normally doesn't use Ansible's "become" system at all.  Some tasks,
though, need to to use it to switch to a specific unprivileged user.
For those, `doas` is not appropriate, since it doesn't permit root by
default.  Rather than adjust this configuration, let's switch to `su`
which requires no configuration, and does allow root.

host_provisioner.py

@@ -63,7 +63,7 @@ def amqp_connect() -> pika.BlockingConnection:
 
 
 def apply_playbook(*args: str) -> None:
-    cmd = ['ansible-playbook', '-u', 'root']
+    cmd = ['ansible-playbook', '-u', 'root', '--become-method', 'su']
     cmd += args
     log.debug('Running command: %s', cmd)
     subprocess.run(cmd, check=True, stdin=subprocess.DEVNULL)