Add gevent based Python server to benchmark test cases (#794)

This add the benchmark test cases, which is using gevent based on wsgi
framework.

test/benchmark/assets/envs/python_gevent/env.yaml

@@ -0,0 +1,23 @@
+apiVersion: fission.io/v1
+kind: Environment
+metadata:
+  name: python
+  namespace: default
+spec:
+  version: 2
+  poolsize: 3
+  TerminationGracePeriod: 5
+  resources:
+    limits:
+      cpu: 300m
+      memory: 256Mi
+    requests:
+      cpu: 300m
+      memory: 256Mi
+  runtime:
+    image: fission/python-env
+    imagePullPolicy: IfNotPresent
+    container:
+      env:
+      - name: WSGI_FRAMEWORK
+        value: "GEVENT"