VMware: Add check for datacenter with no datastore

When there is a datacenter with no datastore in it,
nova boot fails.

This patch addresses the issue by adding a check
and ignoring the empty datacenter.

closes-bug: #1302482
(cherry picked from commit 529e7d14a716fcc1264b2e8053be3c842afd2153)

Conflicts:

	nova/tests/virt/vmwareapi/test_vmops.py

Change-Id: I3c8242767f5991d4aeffab09c763a21493fc363e
Merge "VMware: Add check for datacenter with no datastore" into stable/havana