fixed JcloudsLocation override of groupId causes DnsNameValidator to fail in jclouds

fixed only for vcloud provider...

I get the following exception when attempting to provision a machine in vCloud Director 1.5:

```
Object 'br-shan-aasavh-9e6' doesn't match dns naming constraints. Reason: Can't be null or empty. Length must be 3 to 15 symbols
```

The override of groupId in JcloudsLocation occurs here:

https://github.com/brooklyncentral/brooklyn/blob/master/core/src/main/java/brooklyn/location/basic/jclouds/JcloudsLocation.java#L481

But some extra characters get appended sometime after that (in the case above '-936') which pushes beyond the max allowed 15 char limit in DnsNameValidator.

See note about vCloud Director 1.5 limitation here:

https://github.com/jclouds/jclouds/blob/master/apis/vcloud/src/main/java/org/jclouds/vcloud/compute/strategy/InstantiateVAppTemplateWithGroupEncodedIntoNameThenCustomizeDeployAndPowerOn.java#L95