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/data/demo_testcase_variables.yml

@@ -1,16 +1,16 @@
 - config:
     name: "create user testcases."
     variables:
-        - device_sn: 'HZfFBh6tU59EdXJ'
+        device_sn: 'HZfFBh6tU59EdXJ'
     base_url: ${get_base_url()}
 
 - test:
     name: get token
     variables:
-        - user_agent: 'iOS/10.3'
-        - os_platform: 'ios'
-        - app_version: '2.8.6'
-        - sign: f1219719911caae89ccc301679857ebfda115ca2
+        user_agent: 'iOS/10.3'
+        os_platform: 'ios'
+        app_version: '2.8.6'
+        sign: f1219719911caae89ccc301679857ebfda115ca2
     request:
         url: /api/get-token
         method: POST
@@ -31,8 +31,8 @@
 - test:
     name: create user which does not exist
     variables:
-        - user_name: "user1"
-        - user_password: "123456"
+        user_name: "user1"
+        user_password: "123456"
     request:
         url: /api/users/1000
         method: POST