JBIDE-17031 add urls for 4.2.0.Alpha2 in products.yml

This adds 4.2.0.Alpha2.

A few thing I noticed:

download_url is mentioned a ton of places in products.yml but they all point to jboss.org locations - which eventually would point back to us, plus I can't see it used anywhere - should this get removed too ?

it would be nice if md5_url would just state something like md5_url: - to just assumg it is md5_url: <url>.MD5 (could cut down alot of typing ;)

And how do I enable the central tab ?