Fix bug SSH interactive mode

Commit b44db20 introduced a bug that broke
the SSH interactive mode. If no command is
provided, Driver.GetSSHCommand is called with
an empty string instead of no parameter.

Signed-off-by: Guillaume Giamarchi <guillaume.giamarchi@gmail.com>
Merge pull request #101 from ggiamarchi/fix-interactive-ssh

Fix bug SSH interactive mode