EC: Add 'erasure_coding' policy type, PyEClib dependency

This patch adds support for a new policy type called
'erasure_coding' to lay the groundwork required to
support Erasure Codes in Swift.

Changes:

 - Policy config changes
   - New policy type 'erasure_coding'
   - Add:
     ec_type (EC backend),
     ec_num_data_fragments (number of fragments original
         data split into after erasure coding operation),
     ec_num_parity_fragments (number of redundant parity
         fragments generated during erasure coding)

 - Subclass StoragePolicy into ECStoragePolicy and
   ReplicationStoragePolicy.

 - ECStoragePolicy holds EC specific metadata and ring
   validator methods.

Implements: blueprint ec-proxy-work
Change-Id: Id79eb337176462b43a116812117fb569f4dbe868