T5840: Add override for systemd kea-ctrl-agent.service

After update KEA to 2.4.x in the bf04cd8fea44d375fb7d93d75a1f31c220730c88
there is a file that expects ConditionFileNotEmpty=/etc/kea/kea-api-password
It cause the unit kea-ctrl-agent.service cannot start

systemd[1]: kea-ctrl-agent.service - Kea Control Agent was skipped because of an unmet condition check (ConditionFileNotEmpty=/etc/kea/kea-api-password)

Override systemd kea-ctrl-agent.service do not check this file