[#493] node/config: Fix corrupting of path to the subsection

In previous implementation `Config.Sub` method could lead to the violation
of the internal `path` slice because of `append`. This has been observed on
deeply nested subsections.

Fix `Config.Sub` to copy internal slice in order to prevent violations.
Cover problem case in test config files and unit test.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/config/test/config.json

@@ -1,7 +1,18 @@
 {
   "value": "some value",
+
   "section": {
-    "any": "thing"
+    "any": "thing",
+    "sub": {
+      "sub": {
+        "sub1": {
+          "key": "val1"
+        },
+        "sub2": {
+          "key": "val2"
+        }
+      }
+    }
   },
 
   "string_slice": {

cmd/neofs-node/config/test/config.yaml

@@ -3,6 +3,14 @@ value: some value
 section:
   any: thing
 
+  sub:
+    sub:
+      sub1:
+        key: val1
+
+      sub2:
+        key: val2
+
 string_slice:
   empty: []