MULE-7584 - implement dual-authentication for SFTP Connector

a client requested SFTP connector to use dual-authentication (username with password *AND* SSH key-file).

Change has been implemented and tested. 

See 
http://openssh.com/txt/release-6.2 and 
http://serverfault.com/questions/193198/how-can-i-enable-both-password-and-public-key-authentication-with-openssh

for details