ansible: Change hostname to inventory_hostname

Due to the change caused by updated terraform script, we need to change
usage of variable `hostname` to `inventory_hostname`.

Signed-off-by: markoburcul <marko@status.im>

templates/config/elasticsearch.yml.j2

@@ -1,4 +1,4 @@
-node.name: "{{ hostname }}"
+node.name: "{{ inventory_hostname }}"
 
 # act as "search load balancer", fetching data, aggregating results, etc.
 node.roles: [ ]