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/testsuites/create_users.yml

@@ -1,17 +1,17 @@
 - config:
     name: create users with uid
     variables:
-        - device_sn: ${gen_random_string(15)}
+        device_sn: ${gen_random_string(15)}
     base_url: "http://127.0.0.1:5000"
 
 - test:
     name: create user 1000 and check result.
     testcase: testcases/create_and_check.yml
     variables:
-        - uid: 1000
+        uid: 1000
 
 - test:
     name: create user 1001 and check result.
     testcase: testcases/create_and_check.yml
     variables:
-        - uid: 1001
+        uid: 1001