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/requirements.txt b/requirements.txt
index 70667b6..7a09d68 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,3 +1,4 @@
 paramiko>=1.16.0,!=2.0.1
 PyYAML>=3.1.0
 six>=1.9.0
+cookiecutter