JcloudsLocation: support unsetting config

- if set null loginUser, loginUserPassword, etc then don't pass it through to jclouds (i.e. don't NPE)

Use-case is... If brooklyn.properties contains values for provisioning (e.g. for windows machines), but in the app code you want to provide SoftwareProcess.PROVISIONING_PROPERTIES for a specific entity (e.g. for a linux machine), then want to be able to set some properties back to null, rather than it taking the values picked up from brooklyn.properties for that named location.

We could also guard the other templateOptions to not set them if non-null. If doing that, should we in fact do this in a more generic way rather than having a null-check everywhere?