Show where files are copied for sudo switch to stack user

The home directory for the 'stack' user defaults to /opt/stack, which is
not obvious to devstack newbies, and can also be overridden by exporting
a value for DEST.  Therefore it's friendlier to be explicit about the
location of this home directory, to which devstack is copied before
being run as the 'stack' user.

Change-Id: Ia1941a5f2f8cf86a06681e85da52b817a855b8ff
Merge "Show where files are copied for sudo switch to stack user"