EC: Allow tuning ec_object_segment_size per policy

For 'erasure_coding' type (EC) policies, objects are buffered up to
a defined 'segment size' before they are erasure (en)coded.  There
is a desire to make this size configurable per policy given PyECLib
metadata/padding overheads and thus fragment sizes vary depending on
the EC scheme and ec_num_data_fragments/ec_num_parity_fragments values
chosen.  A cluster administrator setting up an EC policy may be able
to tune the object segment size value for a balance of space savings
and performance for a particular EC scheme.

Changes:

 - Parsing support for 'ec_object_segment_size' swift.conf option
 - Add 'ec_segment_size' property to ECStoragePolicy class
 - Add unit test cases for ec_object_segment_size

Co-Authored-By: Yuan Zhou <yuan.zhou@intel.com>

DocImpact
Implements: blueprint ec-proxy-work
Change-Id: I3a9628b404e3921108c132093162331acb3e73d3
Merge "EC: Allow tuning ec_object_segment_size per policy" into feature/ec