This patch makes the reconciler PPI aware. It does this by adding a
helper method `can_reconcile_policy` that is used to check that the
policies used for the source and destination aren't in the middle of a
PPI (their ring doesn't have next_part_power set).
In order to accomplish this the reconciler has had to include the
POLICIES singleton and grown swift_dir and ring_check_interval config options.
Closes-Bug: #1934314
Change-Id: I78a94dd1be90913a7a75d90850ec5ef4a85be4db
e491693e36