[pytests]: creds fixture does not load correct groupvars creds

[lguohan]

Description
[pytests]: creds fixture does not load correct groupvars

Steps to reproduce the issue:

py.test --inventory veos.vtb --host-pattern all --user admin -vvv --show-capture stdout --testbed vms-kvm-t0 --testbed_file vtestbed.csv --disable_loganalyzer --log-file logs/1vlan/tacacs/test_rw_user.log --junitxml=results/1vlan/tacacs/test_rw_user.xml tacacs/test_rw_user.py

Describe the results you received:

18:39:43  ptfhost = <common.devices.PTFHost object at 0x7fa0e83d7b10>
18:39:43  duthost = <common.devices.SonicHost object at 0x7fa0e9c82710>
18:39:43  creds = {'ceos_image': 'ceosimage:4.23.2F-1', 'ceos_image_orig': 'ceosimage:4.23.2F', 'eos_default_login': 'admin', 'eos_default_password': '', ...}
18:39:43  
18:39:43      @pytest.fixture(scope="module")
18:39:43      def setup_tacacs(ptfhost, duthost, creds):
18:39:43          """setup tacacs client and server"""
18:39:43      
18:39:43          # disable tacacs server
18:39:43          ptfhost.service(name="tacacs_plus", state="stopped")
18:39:43      
18:39:43          # configure tacacs client
18:39:43  >       duthost.shell("sudo config tacacs passkey %s" % creds['tacacs_passkey'])
18:39:43  E       KeyError: 'tacacs_passkey'

Describe the results you expected:
test should pass.

The issue is that current creds fixture use the inventory file name as group name, which is not correct.
Additional information you deem important:

**Output of `show version`:**

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

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