Check for HADOOP_CONF_DIR env var
Check for HADOOP_CONF_DIR env var

When setting CONF_DIR in yarn_api_client/hadoop_conf.py
it would be nice to check for conf dir overrides defined
via the HADOOP_CONF_DIR env var. This comes in handy when
working with hadoop clients defined in non standard paths.

Closes #25

toidi/hadoop-yarn-api-python-client