When accessing nested facts, such as in ansible_local sections, guard
statements in the when clause can get quite long, due to having to
repeate the 'in/not in' logic for every key at every level.
The get_nested filter attempts to instead to move the guard into a
single line, making the conditions easier to write and maintain.
As example,
('openstack_ansible' not in ansible_local or
'swift' not in ansible_local['openstack_ansible'] or
'venv_tag' not in ansible_local['openstack_ansible']['swift'] or
ansible_local['openstack_ansible']['swift']['venv_tag'] == swift_venv_tag)
could be rewritten as
get_nested(ansible_local, 'openstack_ansible.swift.venv_tag') == swift_venv_tag
Change-Id: I3b43c25c8783c43cf5285f2b3e7267b2c5712ea0
80a1262252