scripts: merge-standard-inventory ignore non-existant directory
    
    The merge-standard-inventory script should ignore a non-existant
    directory. This bug was caught by the integration tests.
    
    Otherwise we see this problem:
    
        Traceback (most recent call last):
          File "/data/src/standard-test-roles/scripts/merge-standard-inventory", line 208, in <module>
            sys.exit(main(sys.argv))
          File "/data/src/standard-test-roles/scripts/merge-standard-inventory", line 37, in main
            merged_data = merge_standard_inventories(argv[1:])
          File "/data/src/standard-test-roles/scripts/merge-standard-inventory", line 82, in merge_standard_inventories
            for i in os.listdir(inventory_dir):
        OSError: [Errno 2] No such file or directory: '/usr/share/ansible/inventory'