Initialize datastore_root from cli args in cli.step(...) (#134)

* Initialize datastore_root from cli args in cli.step(...)

Gitter thread - https://gitter.im/metaflow_org/community?at=5e4b8e24d56ddb68a4aff877

Metaflow initializes datastore_root to fetch user code in `step`
command. The behaviour defaults to relying on METAFLOW_DATASTORE_SYSROOT_S3
environment variable to initialize datastore_root. This patch checks if
--datastore_root cli arg can be used instead.

* remove cli.py:769-770

metaflow/cli.py

@@ -401,6 +401,7 @@ def step(obj,
          fg='magenta',
          bold=False)
 
+    obj.datastore.datastore_root = obj.datastore_root
     if obj.datastore.datastore_root is None:
         obj.datastore.datastore_root = obj.datastore.get_datastore_root_from_config(obj.echo)
 

metaflow/datastore/datastore.py

@@ -328,7 +328,7 @@ def __init__(self,
                                 "Try with a different --run-id.")
         if self.datastore_root is None:
             raise DataException("Datastore root not found. "
-                                "Specify with DATASTORE_SYSROOT_%s "
+                                "Specify with METAFLOW_DATASTORE_SYSROOT_%s "
                                 "environment variable." % self.TYPE.upper())
 
         self.event_logger = event_logger