Make ElasticSearchAutoConfiguration conditional on SD Elasticsearch

ElasticSearchAutoConfiguration depends on two Spring Data Elasticsearch
classes (TransportClientFactoryBean and NodeClientFactoryBean), however
it’s only conditional on Elasticsearch itself being on the classpath.
This lead to start up failures due to a ClassNotFoundException. Its
@ConditionalOnClass configuration has been updated so that the
auto-configuration will only be enabled if both Elasticsearch and Spring
Data Elasticsearch are on the classpath.

The dependencies on TransportClientFactoryBean and NodeClientFactoryBean
were ‘hidden’ in ElasticsearchProperties. The logic that uses these
types has been moved into ElasticSearchAutoConfiguration so that the
usage of the types and the related @ConditionalOnClass configuration
is in the same file.

Fixes #1023