s/packerio_package_manager/ansible_pkg_mgr/g

Ansible provides a fact "ansible_pkg_mgr". Use that in favor of
creating a variable ourselfes.
ansible-packerio 0.7.0
Merge branch 'ansible-packerio-0.7.0'

* ansible-packerio-0.7.0:
  ansible-packerio 0.7.0
  s/packerio_package_manager/ansible_pkg_mgr/g