Add service['cassandra'] resource

Have to make Chef aware of the service, so we can properly
manage it
Manage cassandra-env.sh using Chef attributes

Defaults to _not_ restarting Cassandra on changes to
this, and in the future other, config files
Populate topology configs via Chef Search

Search string is configurable w/ an attribute.

By default, search for nodes that have this recipe applied, and
are in the same environment.

Due to Cassandra being infrastructure agnostic, it uses the
more generic terms `DC` (datacenter) and `RAC` (rack) to
denote where a particular node lives.

Some manual notation is needed to translate that to AWS-speak
(i.e., region = datacenter, AZ = rack).

Lastly, there's probably a better way to handle the output,
esp. the YAML, but this works well enough.