Join fails with "Failed to bootstrap the system: SSL Error."

[tserong]

Seen while testing #758. Tried to join a new node to the cluster, and got this:

From a quick grep through src/gravel/controllers/deployment/join.py, it looks like we might be trying to default to https:// when calling /api/deploy/join/request on the existing host, but that's never going to work until #759 goes in.