EC: Make quorum_size() specific to storage policy

quorum size ie, the number of successful backend operations
needed for the proxy to consider the client request
serviceable or successful, depends on the policy type.
For 'replication' policies, it is a function of the number
of object replicas configured, whereas for 'erasure_coding'
policies, it is a function of the number of fragments which
is also the number of nodes that the original data was split
across.

This patch adds quorum_size() definitions for the currently
supported storage policy (sub)classes:
   ReplicationStoragePolicy and ECStoragePolicy.

Changes:

 - Move quorum_size() from utils to *StoragePolicy
 - Update callers of quorum_size() to reflect this change
 - Add quorum_size() unit test code taking policy_type
   into account

Implements: blueprint ec-proxy-work
Change-Id: Ifaf8696e45c42176936c91d3a50b8426ec6bec78