HDDS-2628. Make AuditMessage parameters strongly typed

[adoroszlai]

What changes were proposed in this pull request?

1. Improve type safety in AuditMessage$Builder for methods forOperation and withResult by using existing interface AuditAction and enum AuditEventStatus respectively instead of Strings.
2. Use existing Server.getRemoteAddress() instead of Server.getRemoteIp().getHostAddress() with null check
3. Define and use getRemoteUserName() along the same lines

https://issues.apache.org/jira/browse/HDDS-2628

How was this patch tested?

Created keys using Freon, verified audit log entries.

2019-11-27 20:48:16,206 | INFO  | SCMAudit | user=hadoop | ip=172.23.0.3 | op=ALLOCATE_BLOCK {owner=88982149-2c09-4cd7-8e38-fba8f23cff5e, size=268435456, type=RATIS, factor=ONE} | ret=SUCCESS |
2019-11-27 20:48:45,879 | INFO  | SCMAudit | user=hadoop | ip=172.23.0.4 | op=SEND_HEARTBEAT {datanodeUUID=4e2ee488-6dc6-45f8-9f02-02f1a5cff554, command=[]} | ret=SUCCESS |

2019-11-27 20:48:16,208 | INFO  | OMAudit | user=hadoop | ip=172.23.0.2 | op=ALLOCATE_KEY {volume=vol1, bucket=bucket1, key=OkoBsDwxuj/9, dataSize=10240, replicationType=RATIS, replicationFactor=ONE, keyLocationInfo=[blockID {
  containerBlockID {
    containerID: 1
    localID: 103211840058556425
    ...

https://github.com/adoroszlai/hadoop-ozone/runs/323724573

[dineshchitlangia]

Thanks @adoroszlai for the improvement.

[adoroszlai]

Thanks @dineshchitlangia for reviewing and merging it.