SSE events from HA2 are shown on HA1.

As reported here #426 (comment 169307) by @godfryd:

SSE events from HA2 are shown on HA1.

Repro:

1. Add machines agent-kea-ha1 and agent-kea-ha2
2. Kill DHCPv4 daemon on agent-kea-ha2 using Stork Env Simulator on http://localhost:5000/
3. Observe list of events on Kea app on agent-kea-ha1
4. After a minute or 2 there are automatically added events about Kea from agent-kea-ha2

When the page is refreshed with F5 key then only this Kea events are presented.