provide support for (userName + password + identityFile) authentication

see https://www.mulesoft.org/jira/browse/MULE-7584

Note: I have manually tested the dual (password + identityFile) authentication against a real SFTP server configured with 

AuthenticationMethods publickey,password           //(supported by OpenSSH 6.2+). 

I have not written unit-tests for this because Apache SSHD does not support dual-authentication as far as I can see in the documentation.

See JIRA https://www.mulesoft.org/jira/browse/MULE-7584.