introduce flag to start/stop agent
first define the agent config file and only then the service, for initial creation
Merge branch 'start_stop_agent' of https://github.com/smalltownheroes/chef-datadog into smalltownheroes-start_stop_agent
cleanup for #94

- remove logging statement
- convert conditional into concise ternary
- clean up style
doc update for #90 & #94