Replace dict_key by inventory_hostname

[antifob]

dict_key is defined for each hosts in ansible's inventory. Its value is used to access configuration parameters in a lab's data/config.json file. Since the value of dict_key is always equal to the machine's name in ansible's inventory, it is equal to ansible's inventory_hostname variable.

This patch:

• replaces dict_key by inventory_hostname in playbooks and roles;
• removes dict_key from inventories.