fragment size adjustment in preset via API

[kinvaris]

As part 1 for #204: integration of fragment size adjustment in API

Validated with integration testing:

{u'compression': u'snappy',
  u'fragment_checksum': [u'crc-32c'],
  u'fragment_encryption': [u'none'],
  u'fragment_size': 2097152, <---------------------------------------
  u'in_use': False,
  u'is_available': False,
  u'is_default': False,
  u'name': u'mypreset',
  u'object_checksum': {u'allowed': [[u'none'], [u'sha-1'], [u'crc-32c']],
   u'default': [u'crc-32c'],
   u'verify_upload': True},
  u'osds': [u'all'],
  u'policies': [u'(2, 2, 3, 4)'],
  u'policy_metadata': {u'(2, 2, 3, 4)': {u'in_use': False,
    u'is_active': False,
    u'is_available': False}}}]