Add AES256-GCM-SHA384 to the allowed SSL ciphers.

AWS's Clasic ELB health check tool requires this cipher in order to
proxy to Chef Server nodes.

omnibus/files/private-chef-cookbooks/private-chef/attributes/default.rb

@@ -469,7 +469,7 @@
 # Based off of the Mozilla recommended cipher suite
 # https://mozilla.github.io/server-side-tls/ssl-config-generator/?server=nginx-1.8.1&openssl=1.0.1u&hsts=no&profile=modern
 default['private_chef']['nginx']['ssl_protocols'] = "TLSv1.2"
-default['private_chef']['nginx']['ssl_ciphers'] = "ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:!aNULL:!eNULL:!EXPORT"
+default['private_chef']['nginx']['ssl_ciphers'] = "ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:AES256-GCM-SHA384:!aNULL:!eNULL:!EXPORT"
 #
 # The SSL Certificate and DH Param will be automatically generated if
 # these are nil.  Otherwise we expect these attributes to point at the