Update CFN to allow starting of i2.2xlarge ES nodes

From @mrdavidlaing:

@dpb587, these are the CFN changes I made to the existing cityindex.logsearch.io cluster to enable running i2.2xlarge nodes; specifically

live-logsearch-ElasticsearchDaytimeGroup-AFCJ72SHXWNK
live-logsearch-ElasticsearchFulltimeZone1Group-6SYZH3M84SRR
live-logsearch-ElasticsearchFulltimeZone2Group-1TWREYORFN1AZ
However, I'm struggling to identify the correct source files to update. Please can you help.

Also, I had to update the Ubuntu AMI to one based on the HVM hypervisor, since i2.* nodes don't run on the older PV hypervisor.

This commit is more of a conversation starter than a finished bit of work.