Allow '0' as a valid value for TTL field in DNS static entry

[rimashah25]

What does this PR (Pull Request) do?

• This PR fixes Portal: Cannot use "0" as a TTL value for Static DNS entries #3400

Which Traffic Control components are affected by this PR?

• Traffic Portal
• Traffic Ops

What is the best way to verify this PR?

Manual Testing

1. In Traffic Portal, create a HTTP delivery service
2. Click on More and then add Static DNS entry
3. Select a Type and type ttl value as zero (0)
4. You can create a static DNS entry with a successful message.

Automation Testing

1. Run CIAB
2. Run Traffic_Portal locally (make sure the Traffic_Ops url point to CIAB traffic ops url)
3. Ensure protractor and webdriver-manage are installed.
4. Run webdriver-manager start
5. Run protractor config.js from traffic_control/traffic_portal/test/end-to-end/ folder
6. All 73 specs should pass. 73 specs, 0 failures

If this is a bug fix, what versions of Traffic Control are affected?

• master (e313410)

The following criteria are ALL met by this PR

• This PR includes tests
• This PR does not include documentation
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[mitchell852]

LGTM

[ocket8888]

Works like it says, all unit and client/API integration tests passing.