Disable placement leadership timeouts when there are no peers. This
reduces startup time of placement by a second. Also remove initial delay
of actor health check probe.

Continue to use existing placement config if there is more then one peer
(initial cluster) configured i.e. ha mode.

`-focus "actor|placement" integration test results before and after
change:

```
    integration.go:82: Total integration test execution time: 2m2.7s
    integration.go:82: Total integration test execution time: 2m5.9s
    integration.go:82: Total integration test execution time: 1m36.3s
    integration.go:82: Total integration test execution time: 1m35s
``

Signed-off-by: joshvanl <me@joshvanl.dev>