Create keystore on RPM and Debian package install

When Elasticsearch starts up, it tries to create a keystore if one does
not exist; this is so the keystore can be seeded. With the RPM and
Debian packages, the keystore would be located in
/etc/elasticsearch. This configuration directory is typically not
writable by the elasticsearch user so the Elasticsearch process will not
have permission to create the keystore. Instead, the RPM and Debian
packages should create the keystore (if it does not exist) on package
installation. This commit enables these packages to do that in the
post-install routines.

Relates #26282

distribution/src/main/packaging/scripts/postinst

@@ -103,11 +103,17 @@ chown -R root:elasticsearch /etc/elasticsearch
 chmod 0750 /etc/elasticsearch
 
 if [ -f /etc/default/elasticsearch ]; then
-   chown root:elasticsearch /etc/default/elasticsearch
+    chown root:elasticsearch /etc/default/elasticsearch
 fi
 
 if [ -f /etc/sysconfig/elasticsearch ]; then
-   chown root:elasticsearch /etc/sysconfig/elasticsearch
+    chown root:elasticsearch /etc/sysconfig/elasticsearch
+fi
+
+if [ ! -f "$ES_PATH_CONF"/elasticsearch.keystore ]; then
+    /usr/share/elasticsearch/bin/elasticsearch-keystore create
+    chown root:elasticsearch "$ES_PATH_CONF"/elasticsearch.keystore
+    chmod 660 "$ES_PATH_CONF"/elasticsearch.keystore
 fi
 
 ${scripts.footer}