Updated openstack/openstack
Project: openstack/python-heatclient  8fe0c09e76513829056a5bb69efa10a755414949

Update stack-preview usage text

`--timeout` during stack-preview is only used in certain validation
paths and `--enable-rollback` is not used at all except to be consistent
with stack-create.  This changes the usage text to reflect these
peculiarities.

Change-Id: I9eafad207075f70b1321bb3559cb1363fe526f85