cache-url: turn off -x by default
cache-url: turn off -x by default

I think that generally this is a lot of noise in the logs, as the
internals of cache-url is well tested, so we don't need to trace log
by default.

Change-Id: I25b5a1ec0d8f99691b2b4b62b9fdd537e5a773e4

openstack/diskimage-builder,openstack/diskimage-builder