Endpoints Runtime Release 2.8.0

This release build was produced at 88fc4b8.

Docker image: gcr.io/endpoints-release/endpoints-runtime-serverless:2.8.0

• Apply retry and network_fail_open for failed server response properly (#110)
• Align --cloud_trace_url_override with ESPv1 (#107)
• Support generate_self_signed_certificate (#105)
• Support websocket (#102)
• TLS support for grpcs backend (#95)