Disable dynamic scripting when not  ELASTICSEARCH_CLUSTERED

ticket :  [OSPR-191](https://openedx.atlassian.net/browse/OSPR-191)

Disable dynamic scripting on elasticsearch as it is insecure  when not ELASTICSEARCH_CLUSTERED
@feanil has disabled this but only work when ELASTICSEARCH_CLUSTERED is True.

So  people who install with the default conf (production/devstack vagrant included )will still contain the eval code !It is important to fix , our edx server was hacked by hacker due to this bug.

See: http://bouk.co/blog/elasticsearch-rce/
CVE: CVE-2014-3120