Add 'create_inventory_context' command

The command 'create_inventory_context' can be used to dump all the
nodes from reclass inventory, they names from recalss.storage,
and any additional keys from nodes if needed, for example:

$ reclass-create-inventory-context \
    -d mcp11-ovs-dpdk.local \
    parameters.linux.network.interface \
    parameters.linux.storage
diff --git a/reclass_tools/helpers.py b/reclass_tools/helpers.py
index fcfc564..75e3185 100644
--- a/reclass_tools/helpers.py
+++ b/reclass_tools/helpers.py
@@ -11,6 +11,18 @@
     return data
 
 
+def create_nested_key(data, path=None, value=None):
+    if type(data) is not dict:
+        raise("Use 'dict' object for 'data'")
+    if type(path) is not list:
+        raise("Use 'list' object with key names for 'path'")
+    for key in path[:-1]:
+        if key not in data:
+            data[key] = {}
+        data = data[key]
+    data[path[-1]] = value
+
+
 def remove_nested_key(data, path=None):
     if type(path) is not list:
         raise("Use 'list' object with key names for 'path'")