Fix typos, and add constant for 'N/A'
Fix typos, and add constant for 'N/A'

Also change human_sized() to human_readable() and remove outdated reference in
comment.

ashahi1/docker-volume-vsphere,shivanshu21/docker-volume-vsphere,BaluDontu/docker-volume-vsphere,lipingxue/docker-volume-vsphere,govint/docker-volume-vsphere,shivanshu21/docker-volume-vsphere,luomiao/docker-volume-vsphere,shuklanirdesh82/docker-volume-vsphere,govint/docker-volume-vsphere,BaluDontu/docker-volume-vsphere,ashahi1/docker-volume-vsphere,pshahzeb/vsphere-storage-for-docker,brunotm/docker-volume-vsphere,venilnoronha/docker-volume-vsphere,govint/docker-volume-vsphere,luomiao/docker-volume-vsphere,luomiao/docker-volume-vsphere,venilnoronha/docker-volume-vsphere,brunotm/docker-volume-vsphere,BaluDontu/docker-volume-vsphere,venilnoronha/docker-volume-vsphere,shivanshu21/docker-volume-vsphere,pshahzeb/vsphere-storage-for-docker,shivanshu21/docker-volume-vsphere,BaluDontu/docker-volume-vsphere,govint/docker-volume-vsphere,pshahzeb/vsphere-storage-for-docker,pshahzeb/vsphere-storage-for-docker,shivanshu21/docker-volume-vsphere,lipingxue/docker-volume-vsphere,lipingxue/docker-volume-vsphere,venilnoronha/docker-volume-vsphere,venilnoronha/docker-volume-vsphere,ashahi1/docker-volume-vsphere,ashahi1/docker-volume-vsphere,shivanshu21/docker-volume-vsphere,luomiao/docker-volume-vsphere,lipingxue/docker-volume-vsphere,brunotm/docker-volume-vsphere,BaluDontu/docker-volume-vsphere,venilnoronha/docker-volume-vsphere,BaluDontu/docker-volume-vsphere,brunotm/docker-volume-vsphere,govint/docker-volume-vsphere,brunotm/docker-volume-vsphere,shuklanirdesh82/docker-volume-vsphere,BaluDontu/docker-volume-vsphere,ashahi1/docker-volume-vsphere,pshahzeb/vsphere-storage-for-docker,shuklanirdesh82/docker-volume-vsphere,luomiao/docker-volume-vsphere,shuklanirdesh82/docker-volume-vsphere,brunotm/docker-volume-vsphere,shuklanirdesh82/docker-volume-vsphere,govint/docker-volume-vsphere,shuklanirdesh82/docker-volume-vsphere,luomiao/docker-volume-vsphere,pshahzeb/vsphere-storage-for-docker,lipingxue/docker-volume-vsphere,lipingxue/docker-volume-vsphere,ashahi1/docker-volume-vsphere