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

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.

[Running Build](http://wilson.ci.opscode.us/job/chef-server-12-trigger-ad_hoc/159/downstreambuildview/)

Fixes https://github.com/opscode/opscode-omnibus/issues/497