install missing elasticsearch client

elasticsearch client must be explicitly installed as it's an
optional backend requirement. this patch installs the client when
installing elasticsearch

Change-Id: I534cf0c78ab1fe7d309ef5f808bbe7b5422b403e

pkg/elasticsearch.sh

@@ -77,6 +77,7 @@ function stop_elasticsearch {
 }
 
 function install_elasticsearch {
+    pip_install elasticsearch
     if is_package_installed elasticsearch; then
         echo "Note: elasticsearch was already installed."
         return