Modify the GHCN obs path for snow DA (#220)

Modify the GHCN obs path to use the data from the `prep` subdirectory
under the running directory.

This PR contributes to NOAA-EMC/GDASApp#2019

Co-authored-by: Cory Martin <cory.r.martin@noaa.gov>

Author: jiaruidong2017

observations/snow/ghcn_snow.yaml.j2

@@ -7,7 +7,7 @@
     obsdatain:
       engine:
         type: H5File
-        obsfile: "{{snow_obsdatain_path}}/{{snow_obsdatain_prefix}}{{observation_from_jcb}}.nc"
+        obsfile: "{{snow_prepobs_path}}/{{snow_obsdatain_prefix}}{{observation_from_jcb}}.nc"
         missing file action: warn
     obsdataout:
       engine: