e2e: adds admin interface in extproc test (#339)

**Commit Message**

This will be helpful in debugging connectivity especially. Now you can
spawn `envoy -c ./tests/extproc/envoy.yaml` and open `localhost:9901` on
your browser to debug the connectivity.

---------

Signed-off-by: Takeshi Yoneda <t.y.mathetake@gmail.com>

.gitignore

@@ -7,6 +7,9 @@ out/
 *for_tests.yaml
 .vscode
 
+# This is the placeholder for the access log file during extproc tests.
+ACCESS_LOG_PATH
+
 # Files and directories to ignore in the site directory
 # dependencies
 site/node_modules

tests/extproc/envoy.yaml

@@ -3,6 +3,12 @@
 # The full text of the Apache license is available in the LICENSE file at
 # the root of the repo.
 
+admin:
+  address:
+    socket_address:
+      address: 127.0.0.1
+      port_value: 9901
+
 static_resources:
   listeners:
     - address: