Allow specification of object devices for audit

In object audit "once" mode we are allowing the user to specify
a sub-set of devices to audit using the "--devices" command-line
option.  The sub-set is specified as a comma-separated list.  This
patch is taken from a larger patch to enable parallel processing
in the object auditor.

We've had to modify recon so that it will work properly with this
change to "once" mode.    We've modified dump_recon_cache()
so that it will store nested dictionaries, in other words it will
store a recon cache entry such as {'key1': {'key2': {...}}}. When
the object auditor is run in "once" mode with "--devices" set the
object_auditor_stats_ALL and ZBF entries look like:
{'object_auditor_stats_ALL': {'disk1disk2..diskn': {...}}}. When
swift-recon is run, it hunts through the nested dicts to find the
appropriate entries.  The object auditor recon cache entries are set
to {} at the beginning of each audit cycle, and individual disk
entries are cleared from cache at the end of each disk's audit cycle.

DocImpact

Change-Id: Icc53dac0a8136f1b2f61d5e08baf7b4fd87c8123
Merge "Allow specification of object devices for audit"