Add basic git repo host keys to known_hosts

[fullyint]

#475 (comment) describes how to populate a remote server's known_hosts via deploy hook. The purpose is to avoid the host key verification failed error during the composer install task.

This PR makes the known_hosts handling part of Trellis core:

• avoids the error for the Vagrant VM (wordpress-install role) too
• users no longer need the deploy hook

I included both rsa and ed25519 key types for GitLab. These work with current master and with the HostKeyAlgorithms that #744 will apply to the SSH client. I avoided the ecdsa type for GitLab because it will be disallowed after #744.

GitHub and Bitbucket seem to only offer the rsa type, before and after #744.

Docs in roots/docs#78

[swalkinshaw]

Good stuff 🚀

[swalkinshaw]

changelog and