Use remote_file to get gpg key used to verify CS add on installs for RHEL5.

Ping @opscode/server-team 

RHEL5 yum does not support ssl protocol SNI, which is what cloudfront uses
which is where the gpg key is stored. Use remote_file to get the key because
it has SNI support, and download the key locally to verify on RHEL5.

[Build](http://wilson.ci.opscode.us/job/chef-server-12-trigger-ad_hoc/156/downstreambuildview/)

Fixes https://github.com/opscode/opscode-omnibus/issues/497
Per @juliandunn's comment, try something like this:

```ruby
key_name = File.basename(remote_gpg_url)
gpg_key = File.join(Chef::Config[:file_cache_path], key_name)
```

This will ensure the key is cached in the cache path of the embedded chef-solo run which is set here:
https://github.com/opscode/opscode-omnibus/blob/master/config/software/private-chef-cookbooks.rb#L28