driver/vmwarevsphere: fix creating VMs on a DRS-enabled cluster
driver/vmwarevsphere: fix creating VMs on a DRS-enabled cluster

Omitting the --vmwarevsphere-hostsystem flag allows the user to create
a VM on a DRS-enabled cluster without specifying the host within the
cluster, allowing DRS to choose the host instead.
Fixes #2893.

Signed-off-by: Andrew Featherstone <c14084d2869e86cc4ffb92685dfbbe85d0c26815@cambridgeconsultants.com>

londoncalling/machine,exoscale/docker-machine,dgageot/machine,ibuildthecloud/machine,dgageot/machine,ibuildthecloud/machine,Herbis/machine,SvenDowideit/machine,docker/machine,voutilad/machine,exoscale/docker-machine,voutilad/machine,docker/machine,londoncalling/machine,dweomer/docker-machine,Herbis/machine,dweomer/docker-machine,SvenDowideit/machine