Do not attempt to retrieve stats before Envoy is running

[gertvdijk]

Describe the bug
Ambassador appears to retrieve Envoy's statistics before it's running. This causes WARNING-level log entries like:

WARNING: EnvoyStats.update failed: HTTPConnectionPool(host='127.0.0.1', port=8001): Max retries exceeded with url: /stats (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f329c1ddc88>: Failed to establish a new connection: [Errno 111] Connection refused',))

(Originally discussed in #1203, requested for reporting individual issue as per comments.)

To Reproduce
Steps to reproduce the behavior:

1. Deploy Ambassador 0.50.1 without any Services for it.
2. Read logs.
3. Find errors about not being able to reach Envoy's /stats.

Expected behavior
No attempt to read Envoy stats if it hasn't started Envoy.

Versions (please complete the following information):

• Ambassador: 0.50.1
• Kubernetes environment not really relevant
• Version not really relevant

[richarddli]

Fixed in 0.50.2