Fixed: mkdir_p creation of directory for $step_tickers_path

Default value: $step_tickers_path = '/etc/ntp/step-tickers',

/etc/ntp is non existing on SLES11 and Ubuntu 12.04 for example. This patch extracts the directory path from $step_tickers_path and creates it with mkdir_p. Now the steptickers file can be created without error.