Bug 18571: Add default ES configuration to koha-conf-site.xml.in

Bug 18571: Add default ES configuration to koha-conf-site.xml.in

This patch adds a default configuration entry for elasticsearch. It will
add localhost:9200 to the server subsection, and koha_instance (replacing instance
for the corresponding instance name) for the namespace.

To test:
- Apply the patch
- Copy the file to the /etc/koha dir:
  $ sudo cp kohaclone/debian/templates/koha-conf-site.xml.in /etc/koha
- Create a new instance:
  $ sudo koha-create --create-db test
=> SUCCESS: /etc/koha/sites/test/koha-conf.xml includes the mentioned section:

 <elasticsearch>
     <server>localhost:9200</server>
     <index_name>koha_test</index_name>
 </elasticsearch>

- Sign off :-D

Note: As the use of ES is syspref driven, this default entry doesn't have any use
until ES is installed and SearchEngine set to Elasticsearch. So it doesn't hurt
but will help end users test the ES integration. Advanced users will take care of
this config entry manually (pointing to external servers/clusters, etc).

Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>