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_parameters.yml

@@ -1,16 +1,16 @@
 - config:
     name: "user management testcase."
     parameters:
-        - user_agent: ["iOS/10.1", "iOS/10.2", "iOS/10.3"]
-        - username-password:
+        user_agent: ["iOS/10.1", "iOS/10.2", "iOS/10.3"]
+        username-password:
             - ["test1","111111"]
             - ["test2","222222"]
     variables:
-        - username: test1
-        - user_agent: "iOS/10.1"
-        - device_sn: ${gen_random_string(15)}
-        - os_platform: 'ios'
-        - app_version: 2.8.5
+        username: test1
+        user_agent: "iOS/10.1"
+        device_sn: ${gen_random_string(15)}
+        os_platform: 'ios'
+        app_version: 2.8.5
     base_url: ${get_base_url()}
     output:
         - token