VMware: fix snapshot failure when host in maintenance mode

The root cause is due to a bug in the VC's handling of the
VirtualDiskManager.DeleteVirtualDisk_Task API, which allows the picking
of any host in a datacenter with access to the datastore participating
in the disk deletion picked be to perform the operation, even when the
host is in maintenance mode and hence will always reject the call when
sent.

The fix uses an alternative API (FileManager.DeleteDatastoreFile_Task)
to delete the vmdk and -flat vmdk files separately. This API does not
suffer from the above-mentioned failure mode.

Closes-Bug: #1229994

(cherry picked from commit 7910385825ccfa705785af360fcd5717656e3557)

Conflicts:
	nova/virt/vmwareapi/fake.py
	nova/virt/vmwareapi/vmops.py

Change-Id: I786365847673e5192a21b654cba951b2e7a6f291
Merge "VMware: fix snapshot failure when host in maintenance mode" into stable/havana