Implement changeMasterServiceMember in ZookeeperConsistencyPersistence
Fix changeMasterServiceMember return type & remove useless override
Merge branch 'master' into clement-change-master

Conflicts:
	nrv-zookeeper/src/main/scala/com/wajam/nrv/zookeeper/consistency/ZookeeperConsistencyPersistence.scala
Merge pull request #285 from wajam/clement-change-master

Implement changeMasterServiceMember in ZookeeperConsistencyPersistence