Add proper PVSCSI controller/disks create() automation (#580)
Add proper PVSCSI controller/disks create() automation (#580)

* With this fix, more than one PVSCSI controller can be created if needed.
It can also find available disk slots on other PVSCSI controllers (not only
the first one) if needed.

* Refactor the code, address the comments from Govindan.

* Address Govindan's comment to reword the error message.

* Address comments from Mark and Prashant.

* Reword the error message.

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