Revert Fix for GCU_DHCP_Relay to resolve HEAD PR

[ridahanif96]

Description of PR

This PR is created to resolve issues in KVM test which are failing in advance utilities HEAD PR. KVM Elastic test are failing in generic_config_updater/test_dhcp_relay.py file due to missing mode attribute.

What is the motivation for this PR?

Modified GCU fixtures/duthost_utilities.py to make it compatible for HEAD PR . We have added "switchport mode command" in duthost_utilites to add mode trunk on Ethernet4 so that it will allow addition of vlan membership on ports. Added a function to check "switchport mode explicitly"

[mssonicbld]

The pre-commit check detected issues in the files touched by this pull request.
The pre-commit check is a mandatory check, please fix detected issues.

Detailed pre-commit check results:
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

tests/common/fixtures/duthost_utils.py:453:1: E303 too many blank lines (3)

flake8...............................................(no files to check)Skipped
check conditional mark sort..........................(no files to check)Skipped

To run the pre-commit checks locally, you can follow below steps:

1. Ensure that default python is python3. In sonic-mgmt docker container, default python is python2. You can run
   the check by activating the python3 virtual environment in sonic-mgmt docker container or outside of sonic-mgmt
   docker container.
2. Ensure that the pre-commit package is installed:

sudo pip install pre-commit

3. Go to repository root folder
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pre-commit install

5. Use pre-commit to check staged file:

pre-commit

6. Alternatively, you can check committed files using:

pre-commit run --from-ref <commit_id> --to-ref <commit_id>

[yejianquan]

LGTM