Support rethinkdbdash's ssl option on each server.

[mishawakerman]

rethinkdbdash supports both of the following config schemas:

{
    db: "some_db",
    pool: true,
    servers: [{
        host: "localhost",
        port: 28015,
        ssl: {
            rejectUnauthorized: false,
        }
    }]
}

and

{
    db: "some_db",
    pool: true,
    servers: [{
        host: "localhost",
        port: 28015,
    }],
    ssl: {
        rejectUnauthorized: false,
    }
}

But only the later is supported by rethinkdb-migrate in its validation of the configuration. This is easy enough to workaround but looks like a small validation oversight in rethinkdb-migrate and ideally we'd pass the same config to rethinkdbdash and rethinkdb-migrate.