SPARK-1898: In deploy.yarn.Client, use YarnClient not YarnClientImpl

https://issues.apache.org/jira/browse/SPARK-1898

Author: Colin Patrick McCabe <cmccabe@cloudera.com>

Closes #850 from cmccabe/master and squashes the following commits:

d66eddc [Colin Patrick McCabe] SPARK-1898: In deploy.yarn.Client, use YarnClient rather than YarnClientImpl