Check for config file existance before loading.

Previously if omniship.yml did not exist in config folder of application
an Errno::ENOENT error would be raised. This change checks for file
existance before attempting to load_file. This is needed so settings can be
passed as options to carrier, without the need for a config file.

Change-Id: I0c77708250b147b3825ea53b1a9dd1bd34b9e4f0