HIVE-5167: webhcat_config.sh checks for env variables being set before sourcing webhcat-env.sh (Thejas M Nair via Daniel Dai, Thejas Nair)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/branch-0.12@1524258 13f79535-47bb-0310-9956-ffa450edef68