Provide unset command

[nhambletCCRI]

It seems like it could be convenient to, instead of requiring the user to unset some collection of env vars by hand (e.g., CLOUD_HOME, HADOOP_HOME, etc), provide a command which unsets the ones that probably matter.

The following might be a reasonable start, and/or overkill:
unset ACCUMULO_HOME CLOUD_HOME HADOOP_COMMON_HOME HADOOP_HDFS_HOME HADOOP_HOME HADOOP_YARN_HOME KAFKA_HOME YARN_HOME ZOOKEEPER_HOME

[vpipkt]

I think the commit below removed the need to unset variables in most users environments (with ccri/dev_env). Can we test that works as intended and update the readme?

2a86943