The 'Create authorized keys file from host vars' task assumes that every
host within the nova_compute group is available. If any host within that
group is offline, excluded through a limit, or has otherwise failed an
earlier task, it will lead to this task failing for every host.
Add a when clause checking for the existence of the nova_pubkey hostvar
so that the task and remainder of the role can continue when a
nova_compute host is unavailable.
Related-Bug: 1531783
Change-Id: Idec01d13c35632204a2ae0f0a848a0ccc5b34d73