Fix the indents of v3 API sample docs

There are some indent typos in v3 API sample docs.
Most API samples' indents are 4 spaces, but some samples' indents
are 2 spaces. This patch fixes them.

These typos have been found during the work for auto-generating API
API sample docs from API validation schemas. The auto-generator'll
output API sample docs with json.dumps() method and that would make
the difference between current docs which are fixed with this patch
because the generator would make API samples consistent.
This patch's purpose is also for preparing the generator.

Change-Id: Ieb6e587fcf4c01c128199950db93e1087bc43f41

doc/v3/api_samples/os-aggregates/aggregate-add-host-post-req.json

@@ -1,6 +1,5 @@
 {
-  "add_host":
-    {
-      "host": "21549b2f665945baaa7101926a00143c"
+    "add_host": {
+        "host": "21549b2f665945baaa7101926a00143c"
     }
-}
+}

doc/v3/api_samples/os-aggregates/aggregate-remove-host-post-req.json

@@ -1,6 +1,5 @@
 {
-  "remove_host":
-    {
-      "host": "bf1454b3d71145d49fca2101c56c728d"
+    "remove_host": {
+        "host": "bf1454b3d71145d49fca2101c56c728d"
     }
-}
+}

nova/tests/integrated/v3/api_samples/os-aggregates/aggregate-add-host-post-req.json.tpl

@@ -1,6 +1,5 @@
 {
-  "add_host":
-    {
-      "host": "%(host_name)s"
+    "add_host": {
+        "host": "%(host_name)s"
     }
 }

nova/tests/integrated/v3/api_samples/os-aggregates/aggregate-remove-host-post-req.json.tpl

@@ -1,6 +1,5 @@
 {
-  "remove_host":
-    {
-      "host": "%(host_name)s"
+    "remove_host": {
+        "host": "%(host_name)s"
     }
 }