Make AMI tag filter optional

[orangejulius]

Right now, it is required to set some sort of tag on the AMIs built with packer-elasticsearch before they can be used with this repo.

It would be convenient if that wasn't required, so people could get started with an AMI right away. However, this code is a bit brittle and seems to require a value. Maybe we can set the default to *?

terraform-elasticsearch/ami.tf

Line 10 in 13a1543

values = ["${var.ami_env_tag_filter}"]