Use ssh key from settings

This PR changes travis build to get `ssh_key` from payload, not from job's config. That way we can decide which key to send in travis hub. Currently I don't use `source`, because we can't distinguish between a key from a config file and a key from the DB.
I find this part a bit confusing. I see that `|base64 --decode` is selectively applied, but it seems to me that it is cleaner to wrap this logic in a separate method.