Expand user when caching files for devstack

* modules/openstack_project/files/nodepool/scripts/devstack-cache.py:
Since subprocess.Popen doesn't expand user when the default
shell=False is in effect, make a separate os.path.expanduser call to
resolve the destination path for caching downloads.

Change-Id: Ia0024d03ea23f201f6e88ba14ff6b478f32c0684