Use $TRIPLEO_ROOT/seed.qcow2 as the cache.

Removes the use of .cache/image-create/image-file-cache/{hash}
as the seed image cache. This makes devtest_seed.sh
consistent with the other devtest scripts in that
images get cached in known locations in $TRIPLEO_ROOT.

Change-Id: I28e26d68d66991d26b59dac936a3da63ead8bacf
Merge "Use $TRIPLEO_ROOT/seed.qcow2 as the cache."