## What is the purpose of the change

The hadoop-free distribution of Flink currently does not work on mesos, as the 
call to 
```HadoopUserOverlay.newBuilder().fromEnvironment(configuration).build()``` 
fails with a class not found exception. This change guards against that error 
by checking for Hadoop on the classpath when loading UserGroupInformation; 
modeled after HadoopModuleFactory.

## Brief change log

- *Updated HadoopUserOverlay to handle case where Hadoop is not on CP, without 
breaking API*

## Verifying this change

This change can be verified as follows:

  - *Manually verified the change by starting a Flink Appmaster instance in 
mesos, without the hadoop uber jar in the lib dir*

## Does this pull request potentially affect one of the following parts:

  - Dependencies (does it add or upgrade a dependency): no
  - The public API, i.e., is any changed class annotated with 
`@Public(Evolving)`: no
  - The serializers: no
  - The runtime per-record code paths (performance sensitive): no
  - Anything that affects deployment or recovery: JobManager (and its 
components), Checkpointing, Yarn/Mesos, ZooKeeper: yes
  - The S3 file system connector: no

## Documentation

  - Does this pull request introduce a new feature? no
  - If yes, how is the feature documented? not applicable


[ Full content available at: https://github.com/apache/flink/pull/6689 ]
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