Introduce 'play_hosts_all' next to 'play_hosts'

`play_hosts` are all (active) hosts in the current serialized play.
`play_hosts_all` (*now*) are all (active) hosts in the full play = all serial batches.

use case: in context of deploying clusters of 2 nodes with `serial: 1`, performing a pre-deploy healtcheck, only if the peer node is part of the (full) play: `when: peer_hostname in play_hosts_all`

`play_hosts` didn't suit, as it only contains the serialized batch in a play.