Ensure that the chef shutdown service is configurable

[JonathanSerafini]

New attributes in attributes/cfn-shutdown.rb

# Name of the node which should be deleted when the service executes
#
# This option supports the following values:
# - DETECT_CLIENTNAME: grep the node_name from /etc/chef/client.rb
# - DETECT_NODENAME: node.name
# - DETECT_HOSTNAME: hostname --fqdn
# - *: Any other value is considered as the node_name you wish to delete
#
# @since 2.2.0
default['cfn']['shutdown']['node_name'] = 'DETECT_NODENAME'

Pull request to resolve issue in #1
@webframp @sandervanburken Would this change work for you ?

[webframp]

Is this expected to be the value of the attribute?

[webframp]

Does this need a variables parameter to pass node['cfn']['shutdown']['node_name'] to the template as node_name ?