Modify Remote::FS interface:

* Accept SFTP session in FS constructor;
* Modify Remote to build the session required by FS;
* Change TestEnvHelpers#build_remote in order to provided a test double
  as the remote FS.
Refactor Remote::FS specs
Merge branch 'remote-fs-di'

  Move SFTP session building responsability from Remote::FS class to
Remote class, to decrease coupling in Remote::FS.