Support post-fetching associations

[abellotti]

While the recommended way to get additional attributes and associations is to ask for them ahead of time, i.e.:

   miq.vms.select(:name,:vendor,:disks).where(:name => "dev*").collect(&:disks)

would be nice to be able to post-fetch these, maybe minimally associations, but there will be a performance hit though, ie.:

   miq.vms.where(:name => "dev*").collect(&:disks)

[abellotti]

/cc @mkanoor looks like this issue was already here, I'll close the other one.