Changed the ssh connection to use Puppet's network_device ssh.

This allows us to connect directly to the serial console, and directly set things on the switches, rather than having to SSH into it, which is more unreliable and requires us to know credentials/IP, which we could be changing at any point.
Do we know that we have an MXL versus IOA at this stage? Does it matter? Are there other classes of error from racadm_set_root_creds where we shouldn't be calling set_mxl_root?

I'm guessing it doesn't matter that much, that the set_mxl_root will just fail if it's not an MXL.