ECStoragePolicy.fragment_size is never changed on running Swift because
it is from ec_segment_size and ec_type defined in swift.conf statically
so let's cache the value after retrieving the value from the pyeclib driver.
And more, pyeclib <= 1.2.1 (current newest) has a bug [1] to leak the reference
count of the items in the returned dict (i.e. causes memory leak) so that
this caching will be mitigation of the memory leak because this saves the call
count fewer than current as possible.
Note that the complete fix for the memory leak for pyeclib is proposed at
https://review.openstack.org/#/c/344066/
1: https://bugs.launchpad.net/pyeclib/+bug/1604335
Related-Bug: #1604335
Change-Id: I6bbaa4063dc462383c949764b6567b2bee233689
2876f59d4c