ssconf: Add dry-run support for writing files

A new utility for configuring the node daemon will support a dry-run
mode. This patch adds the necessary functionality to
“ssconf.SimpleStore” and provides comprehensive tests for
“SimpleStore.WriteFiles”. To enable the latter, a testing-only parameter
specifying the lockfile was added to the class.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>