Download improvements

Hello Paul,

Thanks a lot. Your OpenAM vagrant code helped me a lot to get started
on a proof of concept using SAML.

Here are a couple of minor tweaks to the code that downloads OpenAM.
The first patch fixes an issue I ran into where the download timed out.
The second one, caches the OpenAM zip file, so it can be reused if
one has to recreate the environment or run multiple OpenAM instances.