formatter: add the ability to dump attrs in xml entities

xml entities may have attrs assigned to them. Add the ability
to set them. A usage example:

formatter->open_array_section_with_attrs("container",
     FormatterAttrs("name", "foo", NULL));

This will generate the following xml entity:
<container name="foo">

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>

Conflicts:
	src/common/Formatter.cc
rgw: set attrs on various list bucket xml results (swift)

Fixes: #4247
The list buckets operation was missing some attrs on the different
xml result entities. This fixes it.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 2b2704180a12153dc692dd0902515101502a85ba)