Consolidate client vms and standard image/changes in `$GS_HOME/shared/clients` dir

[dalehenrich]

With the new features coming in with #93 and hpi-swa/smalltalkCI#105, it doesn't make much sense to have a separate vm for each client ... especially for the more stable releases like Pharo3.0 and Pharo4.0 --- for the unstable releases, it might make sense to isolate the vm and changes file by client, so it easier to manage multiple versions of the same client using the current mechanism ...

This should be hooked up to the smalltalkCI cache in some way --- see hpi-swa/smalltalkCI#121