Additional change required for Cinder AZ options

This commit adds a check to the dynamic inventory script when parsing extra values.
Presently the sciprt expects a dict, however if the user passes a key with a string value 
the inventory parser breaks. This PR simply adds a check to the scrip to ensure that this
does not happen.

Related Issue: https://github.com/rcbops/ansible-lxc-rpc/issues/458