Add Cassandra support

Add auto-configuration support and health checks for Cassandra and
Spring Data Cassandra.

Fixes gh-2064
Closes gh-2214

spring-boot-samples/spring-boot-sample-data-cassandra/src/test/resources/setup.cql

@@ -0,0 +1,3 @@
+CREATE TABLE customer (id TimeUUID PRIMARY KEY, firstname text, lastname text);
+CREATE INDEX customerfistnameindex ON customer (firstname);
+CREATE INDEX customersecondnameindex ON customer (lastname);