refactor variable context:

1, variable context has two level, testset and testcase;
2, testset level variables can be used in whole test suite, while testcase level variables can only be used in testcase;
3, when variable binds with functions, the funtions will be called and the result will be set to the variable.

tests/data/demo_template_sets.yml

@@ -9,9 +9,6 @@
         gen_md5: "lambda *str_args: hashlib.md5(''.join(str_args).encode('utf-8')).hexdigest()"
     variable_binds:
         - TOKEN: debugtalk
-        - data: ""
-        - random: ${gen_random_string(5)}
-        - authorization: ${gen_md5($TOKEN, $data, $random)}
     request:
         base_url: http://127.0.0.1:5000
 
@@ -19,6 +16,8 @@
     name: create user which does not exist
     variable_binds:
         - data: '{"name": "user", "password": "123456"}'
+        - random: ${gen_random_string(5)}
+        - authorization: ${gen_md5($TOKEN, $data, $random)}
     request:
         url: /api/users/1000
         method: POST
@@ -35,6 +34,8 @@
     name: create user which does exist
     variable_binds:
         - data: '{"name": "user", "password": "123456"}'
+        - random: ${gen_random_string(5)}
+        - authorization: ${gen_md5($TOKEN, $data, $random)}
         - expected_status_code: 500
     request:
         url: /api/users/1000