Restore optional DatacenterFlag
Restore optional DatacenterFlag

The datacenter flag is optional for some commands, such as 'govc ls'.
Revert the portion of a5e26981 that undid this behavior.

austinlparker/govmomi,mevansam/vsphere-govmomi,egorovv/govmomi,vmware/govmomi,egorovv/govmomi,frapposelli/govmomi,ilikejam/govmomi,cgtexmex/govmomi,jrmycanady/govmomi,ilikejam/govmomi,casualjim/govmomi,RotatingFans/govmomi,bruceadowns/govmomi,jrmycanady/govmomi,dougm/govmomi,mevansam/vsphere-govmomi,ilikejam/govmomi,RotatingFans/govmomi,cgtexmex/govmomi,jrmycanady/govmomi,vmware/govmomi,egorovv/govmomi,mevansam/vsphere-govmomi,caglar10ur/govmomi,casualjim/govmomi,bruceadowns/govmomi,caglar10ur/govmomi,cgtexmex/govmomi,dougm/govmomi,frapposelli/govmomi,frapposelli/govmomi,vmware/govmomi,RotatingFans/govmomi,austinlparker/govmomi,caglar10ur/govmomi,bruceadowns/govmomi,dougm/govmomi,casualjim/govmomi,austinlparker/govmomi