Instance groups: only display valid instances for policy members

When displaying the members we will validate that the members
are actually valid. There may be cases when the members of an instance
group are not all valid, for example the instance has been deleted.

This is part of blueprint instance-group-api-extension.

Change-Id: I77062ce14d337662254ca3be956620dda3f44cd7
Merge "Instance groups: only display valid instances for policy members"