make variables in testcase template compatible with mapping style:

new mapping style:
"variables": {
    "a": 1,
    "b": 2
}

Also, the former list style is still valid:
"variables": [
    {"a": 1},
    {"b": 2}
]

tests/testcases/create_and_check.yml

@@ -3,8 +3,8 @@
     name: "create user and check result."
     id: create_and_check
     variables:
-        - uid: 9001
-        - device_sn: "TESTCASE_CREATE_XXX"
+        uid: 9001
+        device_sn: "TESTCASE_CREATE_XXX"
     base_url: "http://127.0.0.1:5000"
 
 - test:
@@ -17,8 +17,8 @@
     name: make sure user $uid does not exist
     api: get_user
     variables:
-        - uid: $uid
-        - token: $token
+        uid: $uid
+        token: $token
     validate:
         - eq: ["status_code", 404]
         - eq: ["content.success", false]
@@ -27,10 +27,10 @@
     name: create user $uid
     api: create_user
     variables:
-        - user_name: "user1"
-        - user_password: "123456"
-        - uid: $uid
-        - token: $token
+        user_name: "user1"
+        user_password: "123456"
+        uid: $uid
+        token: $token
     validate:
         - eq: ["status_code", 201]
         - eq: ["content.success", true]
@@ -39,8 +39,8 @@
     name: check if user $uid exists
     api: get_user
     variables:
-        - uid: $uid
-        - token: $token
+        uid: $uid
+        token: $token
     validate:
         - eq: ["status_code", 200]
         - eq: ["content.success", true]