Make .expiring_objects account name configurable

The account which tracks objects scheduled for deletion had its name
hard-coded to 'expiring_objects'. This is made configurable via
expiring_objects_account_name option. Backend file-systems
integration efforts may want to treat these "special" accounts in a
different way.

This would still go undocumented, hence 'pseudo-hidden'.

UpgradeImpact: None as the default value would continue to be the same
               which is '.expiring_objects'.

Change-Id: I1a093b0d0e2bdd0c3d723090af03fc0adf2ad7e3
Signed-off-by: Prashanth Pai <ppai@redhat.com>
Merge "Make .expiring_objects account name configurable"