Do not use CONF as a default parameter value

In the HashRing.__init__() the "replicas" parameter defaults to
CONF.hash_distribution_replicas, the problem with this approach is
that Ironic might not have loaded the configuration at the time this
configuration is evaluated in the function definition. This patch fix
the problem by using a placeholder value instead of the configuration
option in the function definition.

Partial-Bug: #1279774
Change-Id: I5d8d7cd18a8a6b050f3aa448a337e0156e67ced5
Merge "Do not use CONF as a default parameter value"