Fix HAProxy prechecks during scale-out with limit

Deploy HAProxy on one or more servers. Add another server to the
inventory in the haproxy group, and run the following:

kolla-ansible prechecks --limit <new host>

The following task will fail:

    TASK [haproxy : Checking if kolla_internal_vip_address and
    kolla_external_vip_address are not pingable from any node]

This happens because ansible does not execute on hosts where
haproxy/keepalived is running, and therefore does not know that the VIP
should be active.

This change skips VIP prechecks when not all HAProxy hosts are in the
play.

Closes-Bug: #1868986

Change-Id: Ifbc73806b768f76f803ab01c115a9e5c2e2492ac