Add `discovery.type` into `elasticsearch.yml`

Printing `discovery.type` in the elasticsearch configuration allows the `ec2` type to be enabled
by a data bag or other configuration by means of the `aws` recipe.

Closes #67
Added support for instance tag constraints in EC2 Discovery

The `aws` data bag can now have a structure at `discovery.ec2.tag` that contains EC2 instance tag
values keyed by tag name. Any tag specified will be required for an EC2 instance to
be discoverable by the plugin.

Closes #66