Add support for update_tasks

Add support for update_tasks

These work the same way as upgrade_tasks *but* they use a step variable
instead of tags, so we can iterate over a count/sequence which isn't
possibly via a wrapper playbook with tags (we may want to align upgrade
tasks with the same approach if this works out well).

Note the tasks can be run via ansible-playbook on the undercloud, like:

openstack overcloud config download --config-dir tmpconfig
cd tmpconfig/tripleo-HCrDA6-config
ansible-playbook -b -i /usr/bin/tripleo-ansible-inventory update_steps_playbook.yaml --limit controller

The above will do a rolling update for the Controller role (note the inconsistent
capitalization, we probably need to fix the group naming in tripleo-ansible-inventory)
because we specify serial: 1 in the playbook.

You can also trigger an update explicitly on one node like this, which is useful for debugging:

ansible-playbook -vvv -b -i /usr/bin/tripleo-ansible-inventory update_steps_playbook.yaml --limit overcloud-controller-0

Change-Id: I20bb3e26ab9d9cadf1a31fd304de8a014a901aa9