Implements these pollsters for swift :
- storage.objects (per-tenant number of objects)
- storage.objects.size (per-tenant total size of stored objects)
- storage.objects.containers (per-tenant number of containers)
Implements: blueprint pollster-swift
Fixes: bug #1004450
Change-Id: Ie27abd0b5803043f06899f14b394f2a7ab691bca