rgw: extend manifest to avoid old style manifest

rgw: extend manifest to avoid old style manifest

In case we hit issue #8269 we'd like to avoid creating an old style
manifest. Since we need to have parts that use different prefix we add a
new rule param that overrides the manifest prefix.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 545d8ad1d2dfe53b967ab1bd17dfeb1780abbb5e)