remove debugtalk.py variables mechanism

tests/httpbin/hooks.yml

@@ -1,7 +1,7 @@
 - config:
     name: basic test with httpbin
     request:
-        base_url: $HTTPBIN_SERVER
+        base_url: ${get_httpbin_server()}
     setup_hooks:
         - ${hook_print(setup)}
     teardown_hooks:
@@ -19,7 +19,7 @@
         - ${teardown_hook_sleep_N_secs($response, 1)}
     validate:
         - eq: ["status_code", 200]
-        - contained_by: [content.headers.Host, $HTTPBIN_SERVER]
+        - contained_by: [content.headers.Host, "${get_httpbin_server()}"]
 
 - test:
     name: alter response

tests/httpbin/load_image.yml

@@ -1,7 +1,7 @@
 - config:
     name: load images
     request:
-        base_url: $HTTPBIN_SERVER
+        base_url: ${get_httpbin_server()}
 
 - test:
     name: get png image

tests/httpbin/upload.yml

@@ -1,7 +1,7 @@
 - config:
     name: test upload file with httpbin
     request:
-        base_url: $HTTPBIN_SERVER
+        base_url: ${get_httpbin_server()}
 
 - test:
     name: upload file