get_external_config seems not to read config from $HADOOP_HOME/etc

[visualskyrim]

In the source code here:

snakebite/snakebite/config.py

Line 133 in 6a456e6

def get_external_config(cls):

get_external_conf actually doesn't read core-site.xml or hdfs-site.xml from $HADOOP_HOME/etc.

But in hdfs_try_paths here, snakebite says it tried.

[visualskyrim]

Sorry, I didn't notice that snakebite is using HADOOP_CONF_HOME too.

My HADOOP_CONF_HOME is set to $HADOOP_HOME/etc/hadoop, which makes me think snakebite is reading $HADOOP/etc.

Since snakebite is using HADOOP_CONF_HOME to find hdfs-site.xml and core-site.xml, I think it would be better to have that in the document.

I created another issue to cover this: #241