Update system_kernel stage to use kernel .debs

Signed-off-by: caleb miles <cmiles@pivotallabs.com>

Conflicts:
	stemcell_builder/stages/system_kernel/apply.sh
Fix system_kernel stage

We weren't specifying the virtual Linux kernel image

Signed-off-by: caleb miles <cmiles@pivotallabs.com>
Further fixes for system_kernel stage