Revert "Return isolation methods as a part of listPhysicalNetworks call"

This reverts commit d47a11a7ebcc71af08c958c0d072e011a88720a9.
CLOUDSTACK-6474: IAM - Not able to list shared networks that is created with scope="all"

Changes:
- On startup the root domain group should be created
- Also the SYSTEM and Root Admin accounts should be added to that group
- This will make sure that the root domain shared network's policy gets attached to the root domain group