Bug 26309: Make cxn_pool configurable

Bug 26309: Make cxn_pool configurable

In get_elasticsearch_params we set the conf to static if undefined,
but we never defined it

To test:
1 - Apply unit test patch
2 - prove -v t/Koha/SearchEngine/Elasticsearch.t
3 - It fails
4 - Apply this patch
5 - It succeeds

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>