generate a new TLS cert pair for btcsim

Removes dependency on local btcd cert pair and also fixes tests on travis.
I would save these to variables instead of calling them each time.  Using a switch here would also be a bit cleaner:

```Go
switch {
case haveCert && !haveKey:
    // handle it
case !haveCert && haveKey:
    // handle it
case !haveCert: // both are missing
    // create them
}
```