Remove echo debug line from slave service script

This removes what looks like an errant debug echo from the Debian slave
service script.
Merge pull request #158 from dealnews/slave_service_echo_fix

Remove echo debug line from slave service script
Install swarm-client 1.16

This is necessary to avoid compatibility issues with newer Jenkins
versions (e.g. see https://issues.jenkins-ci.org/browse/JENKINS-20138).
Merge pull request #162 from mark0n/swarmVersion

bump up default version of swarm client
Add ensure parameter to jenkins::slave

Allows running the slave manually instead of a service, useful in Docker for instance