rgw: allow multisite specification used w/ rgw-restore-bucket-index script

rgw: allow multisite specification used w/ rgw-restore-bucket-index script

When the metadata for a bucket is requested only the default
realm/zonegroup/zone is currently supported. This adds three new
command-line options to rgw-restore-bucket-index:

    -r <realm-name>
    -g <zonegroup-name>
    -z <zone-name>

The multisite specification will then be used in invocations of
`radosgw-admin`, such as to query the zone, get metadata, and invoke
the "object reindex" subcommand.

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