Add config option for # of conductor replicas

Add hash_distribution_replicas option to ironic.common.hash_ring.

The consistent hash ring supports mapping to additional nodes around the
ring if some nodes may be temporarily offline. This option determines
how many nodes should be prepared in advance.

blueprint instance-mapping-by-consistent-hash

Change-Id: I93c2f735e201bbfb266e5a637fc76ddd5ca90e87