Switch sftp service from external to internal
Switch sftp service from external to internal

1) We replace /usr/libexec/sftp-server with internal-sftp
* An in-process sftp server requires no support files when used with ChrootDirectory
* With internal-sftp the login shell is no longer involved

2) Do not configure sftp if user provides empty sftp group

Change-Id: I59389203e12e3fb9304033f8e61a910441386851

fuel-infra/puppet-manifests,fuel-infra/puppet-manifests,fuel-infra/puppet-manifests,fuel-infra/puppet-manifests