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.
Is this specifically targeted at Candlepin dependencies? I wonder if this shouldn't be checked/ensured by the puppet module's of the services that require Java. For example, in our Elasticsearch module we do https://github.com/Katello/puppet-elasticsearch/blob/master/manifests/install.pp#L4