Cleanup work for "AdminCLI change to support multitenancy"

[lipingxue]

The following work are needed as cleanup work for "AdminCLI change to support multitenancy"

• The message printed out when cmd succeeds should be changed to make sure they are consistent with existing AdminCLI command
• The code in this PR AdminCLI change to support multitenancy #645
  assume the "tenant_name" is unique, we should decide how to handle if the "tenant_name"
  is not unique. See @govint comments in AdminCLI change to support multitenancy #645 on the subject as well
• Tests need to be added for error condition , i.e. something that causes auth.py code to throw exceptions
• Also, from AdminCLI change to support multitenancy #645 review:
  • max_size should be validated to be under the datastore size
  • functions dealing with internal DB data structures should be in auth.py
  • tenant delete should not exit on individual vmdk delete failure. It should print & save the info and continue to try to finish the cleanup

[pdhamdhere]

All done with #645 and #729