image cache space check fails when should not

[Kaifeng Wang]

deploy_utils.fetch_image added a check on the space check, but this check is prior to the actual download happens, on cases that the cache does not has enough space but fails early when it shouldn't.
We hit this issue as the cache is a standalone volume has limited space, during concurrent deployment with the same instance image, latter instance failed due to insufficient space.

It appears that the space check should be delayed to when it actually needs more space.

[Jay Faulkner (jason-oldos)]

This is now in the image_cache module, but it looks like a bug of this shape still exists.