Fixes #6278 : Add additional checks to the installer.

This adds two new checks. One is using the output of java --version
to ensure the version is at least 1.7 and is the OpenJDK. The second
is looking for total available disk space of at least 5 GB. This does
not really ensure a space issue, but it is is a good first cut.
> I suppose for now this is a safe check. But when we switch to not installing the packages ahead of time, this check will block installation and we will need to remove it.

We will find out very soon :-)

Or it can only fail if Java is installed and in a wrong version. If it
is not present, it would pass (we will always install the correct
version).

^ Clever, isn't it? :-D

-- 
Later,

 Lukas "lzap" Zapletal
 irc: lzap #theforeman