Add feature to clean up iptable rules after test is done or topology

[mingchik]

tears down

Fix:

1. Add feature to skip retries (3) when nmcli disabled (state = NoneType)
2. Cleans iptables after test is down or tear down topology (happy-state-delete)
3. Able to handle existing iptables rules, will not add again.
4. Remove all the existed iptables rules after tear down.
5. Raises meaningful error message when insertion failed.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[mingchik]

@googlebot I signed it!

[robszewczyk]

Please fix the commit message:

one line for the title, followed by an empty line

description of the changes, markdown is supported within the commit message. In particular, please improve the readability of the message:

• use parallel grammatical construction for all list entries (e.g. 1. Skip entries when nlcli is disabled, 2. Clean up iptables after test teardown , ... 5. Raise meaningful exceptions ...)
• ensure proper grammar

After addressing the commit message and the code-related comments, please force push to branch.

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.