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.
Actually, it's stupid to base that choice on `source`, I will add `encoded` field to `ssh_key` in worker's API, in the spirit of keeping logic on the backend, not on the client. Wdyt?