Revert "Fail if the Overcloud inventory is empty"

This reverts commit 773bef1c7a.

The reverted commit made the inventory fail if there was no overcloud
deployed. However, the inventory is also used a lot in cases with no
overcloud (many validations for instance).

Change-Id: I19a4e02d25037f54c2449396ea84492b49555fd6

scripts/tripleo-ansible-inventory

@@ -138,14 +138,6 @@ def main():
         ansible_ssh_user=configs.ansible_ssh_user,
         plan_name=configs.stack or configs.plan)
 
-    if inventory.get_overcloud_environment() == {}:
-        print("ERROR: The environment data can not "
-              "be empty '{{}}', please check that the --stack "
-              "or --plan options are configured correctly "
-              "and the stack {} exists".format(configs.stack or configs.plan)
-              ,file=sys.stderr)
-        sys.exit(1)
-
     if configs.list:
         try:
             inventory_list = inventory.list()