Added --oem_pkg oem override option to image_to_vm.sh

This sets the IMG_FORCE_OEM_PACKAGE variable to the supplied string.  If a
':' is present, what follows it gets put in the IMG_FORCE_OEM_USE variable
and what precedes in the former.

_get_vm_opt() has been modified to generally support forced overrides such
as this one, simply set variables named IMG_FORCE_$opt.

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.