ironic_pxe bootstrap should run on all hosts

Most bootstrap actions use the run_once flag to run on a single host
as they typically involve service-global operations such as database
syncs. The ironic_pxe container bootstrap is different, as it copies
pxelinux files into the ironic_pxe Docker volume. This should be done
on all hosts but currently is only done on one host.

This change performs the ironic_pxe bootstrap on all hosts in the
ironic-pxe group.

Change-Id: Iffd34e6ff26a0ba5140e5d477418cc8aebcdac62
Closes-Bug: #1667153

ansible/roles/ironic/tasks/bootstrap.yml

@@ -60,5 +60,4 @@
       - "{{ node_config_directory }}/ironic-pxe/:{{ container_config_directory }}/:ro"
       - "/etc/localtime:/etc/localtime:ro"
       - "ironic_pxe:/tftpboot/"
-  run_once: True
-  delegate_to: "{{ groups['ironic-pxe'][0] }}"
+  when: "{{ inventory_hostname in groups['ironic-pxe'] }}"