Various updates to module

Updated module to include: 
 * Pulled out default param values into etcd::params, to add multi-OS support
 * EL OS support, including InitD script and sysconfig file
 * Add support for etcd Discovery 
 * Removed '-max-cluster-size' config, as deprecated
 * Various other tweaks and 

Pulled in commits from following forks:
 * @intjonathan
 * @relistan
 * @bobtfish
 *  Matthew Finch

Will follow up with further test cases for the EL support.