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_binds.yml

@@ -1,10 +1,4 @@
-register_variables:
-    variable_binds:
-        - TOKEN: "debugtalk"
-        - var: [1, 2, 3]
-        - data: {'name': 'user', 'password': '123456'}
-
-register_template_variables:
+bind_variables:
     variable_binds:
         - TOKEN: "debugtalk"
         - token: $TOKEN