rgw: set length for keystone token validation request

Fixes: #7796
Backport: giany, firefly
Need to set content length to this request, as the server might not
handle a chunked request (even though we don't send anything).

Tested-by: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>