oem/ami: reduce parallelism
oem/ami: reduce parallelism

The EC2 builder doesn't have enough resources to run all of these in parallel.
Instead, copy the PV images and then copy the HVM images.

marineam/coreos-scripts,mjg59/scripts,smilart/scripts,mjg59/scripts,marineam/coreos-scripts,marineam/coreos-scripts,mjg59/scripts,glevand/coreos--scripts,glevand/coreos--scripts,coreos/scripts,smilart/scripts,coreos/scripts,smilart/scripts