Fix mandatory variables check for keepalived_virtual_ipaddress_configs
In case the variable keepalived_virtual_ipaddress_configs
was set, the mandatory variables check would have still be failed.
This fixes the issue by allowing having at least one of the variables to be defined.
This is a follow-up of !16 (merged).
@christian.hueser.hzdr Please check and merge if it's fine for you :)