version not reliable, adding optional remote_file

Hey Ryan,

Chef server that launched fine using attribute 'chef-server-blueprint/version=11.10' is now failing with:
```
10:28:57: Omnitruck download-server request: http://www.opscode.com/chef/download-server?p=el&pv=6&m=x86_64&v=11.10&prerelease=false&nightlies=false
*ERROR> Execution failed
*ERROR> Execution error:
Could not locate chef-server package matching version '11.10' for node.

The error occurred line 29 of [COOKBOOKS]//default/3f22227d144b8f9d70ad41f95fb535ee/chef-server/recipes/default.rb in method 'from_file' while executing:

27     err_msg << " nightly" if node['chef-server']['nightlies']
28     err_msg << " package matching version '#{node['chef-server']['version']}' for node."
**     raise err_msg
30   end
31 else

*ERROR> Subprocess exited with 1
```

To make this more reliable, I added attribute 'chef-server-blueprint/remote_file' that can take an HTTP(S) URL(Ex: http://opscode-omnibus-packages.s3.amazonaws.com/el/6/x86_64/chef-server-11.0.12-1.el6.x86_64.rpm) of the chef binary to install. Also, considering the size(~200MB) some might want to drop this in a storage  close to the servers.