[cloud] Do not rely on ECS instances to import images to Alibaba Cloud

[cloud] Do not rely on ECS instances to import images to Alibaba Cloud

Spinning up ECS instances is supported in all ECS regions (unlike
Function Compute), but turns out to be unacceptably unreliable since
Alibaba Cloud has a very irritating tendency to fail to launch ECS
instances for a variety of spurious and unpredictable reasons.

Rewrite the censorship bypass mechanism to use the (extremely slow)
CopyImage API call to copy an imported image from an uncensored region
to a censored region.

Signed-off-by: Michael Brown <mcb30@ipxe.org>