Provide local test facility

[adersberger]

Start Solr Cloud and Zookeeper in-process for unit testing Provide test data

[adersberger]

In a first step it is also OK to:

• provide a local Solr Cloud and Spark with startup scripts and a howto readme
• provide a mock implementation for the Chronix Solr Cloud storage

[adersberger]

Logic to retrieve the shard urls should be moved from ChronixSparkContext to ChronixSolrCloudStorage.