Bug 19542: Add a check for ES configuration health

Bug 19542: Add a check for ES configuration health

This patch adds a new statuc function to Koha::SearchEngine::ElasticSearch
which is instended to replace most of get_elasticsearch_params. This function
reads the configuration from C4::Context->config('elasticsearch') and raises
relevant exceptions when mandatory entries are missing.

Its behaviour is covered by tests.

To test:
- Run:
  $ kshell
 k$ prove t/Koha/SearchEngine/Elasticsearch.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: David Bourgault <david.bourgault@inlibro.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>