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

Author: hansohn

yarn_api_client/hadoop_conf.py

@@ -6,7 +6,7 @@
 except ImportError:
     from http.client import HTTPConnection, OK
 
-CONF_DIR = '/etc/hadoop/conf'
+CONF_DIR = os.getenv('HADOOP_CONF_DIR', '/etc/hadoop/conf')
 
 
 def _get_rm_ids(hadoop_conf_path):