Add new parameter $etcd::manage_service_file

Add a new boolean parameter that determines whether or not this module
will create its own service file.

Useful if your etcd package already provides a service file. Set to
`false` to use the service file provided by your package.
Merge pull request #10 from alphagov/73607174-add_param_to_prevent_managing_service_file

[#73607174] Add new parameter $etcd::manage_service_file