[sonic-py-common] Added an API to get file path containing SONiC version (#6309)

* [sonic-py-common] add an API to get file path containing SONiC version so that the API can be mocked for unit tests.

src/sonic-py-common/sonic_py_common/device_info.py

@@ -297,6 +297,11 @@ def get_sonic_version_info():
 
     return data
 
+def get_sonic_version_file():
+    if not os.path.isfile(SONIC_VERSION_YAML_PATH):
+        return None
+
+    return SONIC_VERSION_YAML_PATH
 
 #
 # Multi-NPU functionality