Update or creation of a notification endpoint deletes the secrets of all existing endpoints

[sranka]

The lastly updated/creating notification endpoint is working fine, other notification endpoints (with secrets) are then failing.

Steps to reproduce:

1. Create 1st notification endpoint (alert tab in UI) with a secret, for example HTTP notification endpoint with Bearer auth method
2. go run ./cmd/influx secret list => OK 1 secret
   060e753a36ab5000-token 28031e0d9c10d773
3. Create 2nd notification endpoint (alert tab in UI) with a secret, for example HTTP notification endpoint with Bearer auth method
4. go run ./cmd/influx secret list => FAIL there just 1 secret, with a new id (for the second endpoint)
   060e761352ab5000-token 28031e0d9c10d773
5. Update 1st notification endpoint in UI with Bearer credentials
6. go run ./cmd/influx secret list => FAIL, there is just 1 secret for the teh 1st notification endpoint

Expected behavior:
There will be 2 secrets in Step 4 and 6

Actual behavior:
Existing secrets are deleted/lost when creating/updating a notification endpoint.

Environment info:
master branch of influxdb as of 2020-07-26