Add 'attributes/customize.rb'.

This file is added to enable improved user experience when overriding
attributes. Today customers often override the complete file, which will
back-fire when OpsWorks introduces new attributes.
By moving out customer attribute changes to separate files, we make it
easier for the customer to keep track of their changes. It is also
easier for OpsWorks to improve the cookbooks.

Once agreed, all cookbooks would have a 'attributes/customize.rb'.
Introduce customize.rb in all cookbooks.
Add keep_releases attribute to opsworks_deploy.

Make it possible to define the number of releases kept. One may
specify this for all apps via "deploy_keep_releases".

  { "opsworks": { "deploy_keep_releases": 5 } }

It is also possible to specify keep_releases for some applications
only.

  { "deploy": { "myapp": { "keep_releases": 5 } } }

Requested via https://github.com/aws/opsworks-cookbooks/issues/103.
Make nginx 'proxy_read_timeout' and 'proxy_send_timeout' configurable
for Unicorn web apps.
use the custom ganglia package versions instead of the package architecture to check if it's installed
Merge remote-tracking branch 'github/master-chef-11.4' into master-chef-11.4