Integrate OfflineMasterKey with a Qubes offlineVM

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The idea is to have a minimal qrexec service that allows the VM running ansible to ask an offlineVM storing Ed25519 master keys for new signing keys.
The offlineVM will generate new Ed25519 signing keys and push them to the ansibleVM.

If the VM running ansible gets compromised, the attacker does not get the Ed25519 master keys (but he can ask for new signing keys).
The key lifetime is configured in the offlineVM and not specified by the ansibleVM.

This involves some initial manual steps when deploying a new relay and does not protect the RSA key (which is on the relay anyway).

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This will start to make (more) sense once tor no longer requires (online) RSA keys.

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An example why this feature is worthwhile to implement:
Ansible security vulnerability allowing target servers to inject commands on the ansible controller host (CVE-2016-8628).