Introduce max fail percentage to playbooks

This allows us to continue execution until a certain proportion of hosts
fail. This can be useful at scale, where failures are common, and
restarting a deployment is time-consuming.

The default max failure percentage is 100, keeping the default
behaviour. A global max failure percentage may be set via
kayobe_max_fail_percentage, and individual playbooks may define a max
failure percentage via <playbook>_max_fail_percentage.

Related Kolla Ansible patch:
https://review.opendev.org/c/openstack/kolla-ansible/+/805598

Change-Id: Ib81c72b63be5765cca664c38141ffc769640cf07

ansible/dell-compute-node-discovery.yml

@@ -7,6 +7,11 @@
 - name: Ensure Dell baremetal compute nodes are PXE booted
   hosts: baremetal-compute
   gather_facts: no
+  max_fail_percentage: >-
+    {{ dell_compute_node_discovery_max_fail_percentage |
+       default(baremetal_compute_max_fail_percentage) |
+       default(kayobe_max_fail_percentage) |
+       default(100) }}
   vars:
     # Set this to the index of the inteface on which to enable PXE.
     drac_pxe_interface: 1