Allow for a null VC uuid for a VM to allow the plugin to be used on standalone hosts.

[govint]

Modified ESX service to allow vc uuid to be zero length string in cases where user has a standalone ESX host with no VC.

Test:

docker volume create -d vsphere vc_vol_1
vc_vol_1
docker volume inspect vc_vol_1
[
{
"Driver": "vsphere",
"Labels": {},
"Mountpoint": "/mnt/vmdk/vc_vol_1/",
"Name": "vc_vol_1",
"Options": {},
"Scope": "global",
"Status": {
"access": "read-write",
"attach-as": "independent_persistent",
"capacity": {
"allocated": "13MB",
"size": "100MB"
},
"clone-from": "None",
"created": "Tue Nov 21 13:02:21 2017",
"created by VM": "master-VM0.0",
"datastore": "sharedVmfs-0",
"diskformat": "thin",
"fstype": "ext4",
"status": "detached"
}
}
]

docker run --rm -it -v vc_vol_1:/mnt centos
Unable to find image 'centos:latest' locally
latest: Pulling from library/centos
d9aaf4d82f24: Pull complete
Digest: sha256:4565fe2dd7f4770e825d4bd9c761a81b26e49cc9e3c9631c58cfc3188be9505a
Status: Downloaded newer image for centos:latest

exit
docker volume rm vc_vol_1
vc_vol_1

[govint]

Test failed because of a host issue, will restart the CI.

from vmware.esxcli.Session import Session, SessionOptions, IsInVisor, \

ImportError: No module named esxcli.Session

[shuklanirdesh82]

LGTM!