In test ecmp/test_fgnhg.py , 'DEFAULT_NAMESPACE' is not defined or imported

[JibinBao]

Description

In the script sonic-mgmt/tests/ecmp/test_fgnhg.py, 'DEFAULT_NAMESPACE' is not defined or imported

Steps to reproduce the issue:

1. In the script sonic-mgmt/tests/ecmp/test_fgnhg.py, search DEFAULT_NAMESPACE
   It is introduced by the PR: [test_fgnhg] Test enhancement for link operational change and matchmode changes to fine grained ECMP #2670

Describe the results you received:
DEFAULT_NAMESPACE is not defined

Describe the results you expected:
DEFAULT_NAMESPACE should be defined or imported

Additional information you deem important:

**Output of `show version`:**

```
(paste your output here)
```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```