Bug 21974: Make Elasticsearch connection settings configurable

Bug 21974: Make Elasticsearch connection settings configurable

Default settings are appended in get_elasticsearch_params so that there's a central place for them. Defaults can be overridden in koha-conf.xml.

Test plan:

1. Verify that searches still work with the patch applied.
2. Verify that search fails after adding the following under <elasticsearch> in koha-conf.xml:
<cxn_pool>Static</cxn_pool>
3. Verify that search fails after adding the following under <elasticsearch> in koha-conf.xml:
<cxn_pool>FOO</cxn_pool>

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>