Added --oem_pkg option to image_to_vm.sh

This option sets the IMG_DEFAULT_OEM_PACKAGE variable to the supplied
string.  If a ':' is present, what follows it gets put in the
IMG_DEFAULT_OEM_USE variable and what precedes in the former.

Now you can do things like:

for fmt in cloudstack \
		digitalocean \
		ec2-compat:ec2 \
		ec2-compat:openstack \
		ec2-compat:brightbox \
		exoscale \
		gce \
		hyperv \
		rackspace \
		rackspace-onmetal; do
	./image_to_vm.sh --format=qemu --oem_pkg=$fmt
	../build/images/amd64-usr/latest/coreos_developer_qemu.sh -curses
done

rather than having to modify build_library/vm_image_util.sh to test oem
builds in qemu.
This is going to have no effect if the vm format defines its a oem package which makes the option a bit misleading. Perhaps turn this function into something that simply validates the value, and modify install_oem_package() so the flag always wins. You could pass the value to install_oem_package with a new global variable like IMG_OEM_PACKAGE and IMG_OEM_USE