Group objects should be removable even if the groups service either doesn't exist or doesn't have the key registered.

[Andy Altepeter]

After upgrading to Silva 2.0, the products refresh step removed the groups service. This cleared all of the defined groups. Not good, since I the group objects were still on the filesystem. I couldn't remove the ip groups, and got the following error:
    * Module ZPublisher.Publish, line 119, in publish
    * Module ZPublisher.mapply, line 88, in mapply
    * Module ZPublisher.Publish, line 42, in call_object
    * Module OFS.ObjectManager, line 524, in manage_delObjects
    * Module OFS.ObjectManager, line 379, in _delObject
    * Module zope.event, line 23, in notify
    * Module zope.component.event, line 26, in dispatch
    * Module zope.component._api, line 130, in subscribers
    * Module zope.component.registry, line 290, in subscribers
    * Module zope.interface.adapter, line 535, in subscribers
    * Module zope.component.event, line 33, in objectEventNotify
    * Module zope.component._api, line 130, in subscribers
    * Module zope.component.registry, line 290, in subscribers
    * Module zope.interface.adapter, line 535, in subscribers
    * Module Products.Silva.IPGroup, line 104, in ipgroup_will_be_removed
    * Module Products.Groups.GroupsService, line 207, in removeIPGroup

To resolve these problems:
1) removeIPGroup shouldn't assume the key exists.
2) The rest of the groups service should be audited to see if the same issue is elsewhere
3) The upgrade shouldn't remove the groups service

Group objects should be removable even if the groups service either doesn't exist or doesn't have the key registered.

[Andy Altepeter (aaltepet)]

Committed fixes for this. This means that the next release of a 2.x Silva should package the latest Trunk version of Groups (which is now 0.6)