Add zenPings(ImmutableList) again

Commit 95c2d844a93f15289be78a479a361c0356e10274 removed `zenPings(ImmutableList<? extends ZenPing> pings)` which actually used by discovery plugins.
For example here: https://github.com/elasticsearch/elasticsearch-cloud-aws/blob/master/src/main/java/org/elasticsearch/discovery/ec2/Ec2Discovery.java#L68

This PR add it back.