use sftp to transfer files instead of scp

@techhat can you take a look at this?

scp is required to be on both sides of the connection in order to work

$ scp this.py salt:
bash: scp: command not found
lost connection

sftp does not, it is actually built directly into the openssh daemon.

http://www.openssh.com/faq.html#3.7
http://www.openssh.com/faq.html#2.10