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/setup.yml

@@ -2,10 +2,10 @@
     name: "setup and reset all."
     id: setup_and_reset
     variables:
-        - user_agent: 'iOS/10.3'
-        - device_sn: "TESTCASE_SETUP_XXX"
-        - os_platform: 'ios'
-        - app_version: '2.8.6'
+        user_agent: 'iOS/10.3'
+        device_sn: "TESTCASE_SETUP_XXX"
+        os_platform: 'ios'
+        app_version: '2.8.6'
     base_url: "http://127.0.0.1:5000"
     verify: False
     output:
@@ -15,10 +15,10 @@
     name: get token (setup)
     api: get_token
     variables:
-        - user_agent: 'iOS/10.3'
-        - device_sn: $device_sn
-        - os_platform: 'ios'
-        - app_version: '2.8.6'
+        user_agent: 'iOS/10.3'
+        device_sn: $device_sn
+        os_platform: 'ios'
+        app_version: '2.8.6'
     extract:
         - token: content.token
     validate:
@@ -29,4 +29,4 @@
     name: reset all users
     api: reset_all
     variables:
-        - token: $token
+        token: $token