[autovalidator] Automatically provision child hotkeys

[ppolewicz]

Automatically, when the user uses a "rotate child hotkey" action (similar to delete) on a ValidatorInstance object:

• destroy the child hotkey for the subnet in question, if any child hotkeys exist (checking on chain, not just in the database)
• create a child hotkey for the subnet in question using the "mother" coldkey (path will be provided in django settings)
• run a install_hotkey.sh script (currently we don't have it yet as another team is running it) from a location which has the subnet codename in it, passing ssh address and local path to the child hotkey
• add the child hotkey to a new Hotkey django model which will be referenced by ValidatorInstance with a ForeignKey, indicating it's from the mother (as opposed to validator hotkey)
• add a celery loop to make sure all hotkeys which should be registered on subnets, are registered on the subnets and if not, run a celery task to register them

TBD by Fel