Docker based demo should use Kea 1.7.8

Our demo setup uses Kea 1.7.4 and Kea 1.7.3 for various agents being installed on docker containers. Kea 1.7.8 introduces new information about HA setup returned in response to the status-get command. To see this information we'd need to upgrade our containers (at least ha1 and ha2) to use Kea 1.7.8.