Adding ability to use zone root records

[smalenfant]

What does this PR (Pull Request) do?

Allowing empty name zone name records.

For example, if your Delivery Service FQDN is test.cdn.isp.net, allow to set an A/AAAA/TXT/CNAME records for that hostname.

At the time of writing this PR, there is no change to the API or Traffic Portal which do not allow empty names. Changes need to be done directly in the database (which does allow empty name).

• is not related to any Issue

Which Traffic Control components are affected by this PR?

• Traffic Router

What is the best way to verify this PR?

This requires to be able to add a static entry in the database using an empty name.

traffic_ops=> select * from staticdnsentry WHERE host = '';
 id  | host |   address   | type | ttl | deliveryservice | cachegroup |         last_updated
-----+------+-------------+------+-----+-----------------+------------+-------------------------------
 146 |      | 10.10.10.10 |   53 | 300 |             437 |            | 2019-11-08 17:59:27.906927+00

Result:

[root@cdn1cdcrs0001 ~]# cat /opt/traffic_router/var/auto-zones/minio.cdn1.coxlab.net.
minio.cdn1.coxlab.net.  300     IN      A       10.10.10.10
minio.cdn1.coxlab.net.  3600    IN      NS      cdn1cdcrs0001.cdn1.coxlab.net.
minio.cdn1.coxlab.net.  3600    IN      NS      cdn1cdcrs0002.cdn1.coxlab.net.
minio.cdn1.coxlab.net.  86400   IN      SOA     cdn1cdcrs0001.cdn1.coxlab.net. traffic_ops.minio.cdn1.coxlab.net. 2019111213 28800 7200 604800 30

Dig verification :

# dig +short @cdn1cdcrs0001.coxlab.net  minio.cdn1.coxlab.net A
10.10.10.10

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

This is an enhancement

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• 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 PR ensures that database migration sequence is correct OR this PR 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

More changes required in Traffic Ops API and Traffic Portal.

[asf-ci]

Can one of the admins verify this patch?

[rawlinp]

Do you mind adding an entry to CHANGELOG.md for this enhancement?

[zrhoffman]

staticDNSEntry.Host validation regex looks good!