Wrapping into lock as we are adding VMs into the dict shared among threads.
Although 'dict' should be thread safe, there seems to be "missing" VMs when scaling down reported by CELAR.  So, added this just in case.
Don't bailout on the failure deleting node instance form the BaseCloudConnector.__vms collection.
Merge branch 'master' of github.com:slipstream/SlipStreamClient