spec: Change summary of glusterfs to distributed file system.

BUG:1146519
Change-Id: I9b24321d9e677b535f8d79691862cf919553d635
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
Reviewed-on: http://review.gluster.org/8844
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>