Added support for manage/unmanage in GPFS NFS driver.
This patch added functions that allow share on Spectrum Scale
node to be managed by OpenStack if existing fileset is an
independent fileset and doesn't have any NFS export
over the fileset path. Also, share can be unmanaged from
OpenStack but still left in Spectrum Scale cluster.
Implements: blueprint gpfs-manage-support
Change-Id: I9134408b59c30ac4bc593f287294741f6e996136