INTSAMPLES-134: SFTP: Use Apache MINA SshServer

JIRA: https://jira.spring.io/browse/INTSAMPLES-134

The `SftpOutboundGatewaySample` doesn't work for me, because of Windows file system.
It isn't eligible to be `LS` by SFTP
We should explain that these are not used for the gateway sample; in fact the gateway sample only works on `localhost`; we either need to fix that, or document it.