rgw: make rgw-restore-bucket-index more robust

rgw: make rgw-restore-bucket-index more robust

This experimental tool write a series of temporary files, the combined
size of which is roughly proportional to some combination of the
number of objects in the bucket and their respective sizes. Previously
the script stored all temporary files in /tmp, which may not be
sufficiently large in many cases.

There are now two enhancements. First, using the "-t" command-line
argument, the directory for temporary files can be specified. Second,
if the script consumes all available space on the file system
containing the temporary directory, an error message to that effect is
displayed, so the user understands what corrective action to take.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>