rebuild: make sure server is shut down before volumes are detached
rebuild: make sure server is shut down before volumes are detached

Currently, we detach block devices before an instance is shut down,
which means all the data, which hasn't been fsynced yet, will possibly
be lost.

Closes-Bug: #1471216
Related-Bug: #1440762

Related upstream commit: 7fe20e96f2604d03ec6a7fa563485ffdcb5cc519

Signed-off-by: blkart <1085ae7ac5d420d52a04a26b37297224b6df9d8b@gmail.com>

eayunstack/nova,eayunstack/nova