feat: use ZEPPELIN_HOSTNAME to specify hostname

[jonbuffington]

ZEPPELIN_HOSTNAME supports running Zeppelin in environments where the
hostname of the server does not match the hostname delivered to the
browser. If ZEPPELIN_HOSTNAME is not set, java.net.InetAddress.
getLocalHost().getHostName() is still used to lookup the server's
current hostname.

NotebookServerTests was renamed to NotebookServerTest to permit the
test cases to run using Maven's test goal.

Fixes https://issues.apache.org/jira/browse/ZEPPELIN-231

[djoelz]

Thanks for fixing my bug :)
I would prefer this setting live in the ZeppelinConfiguration class and not in an environment variable. This way it is discoverable by anyone.

[jonbuffington]

Appreciate the feedback. Do you have any guidance on using ZeppelinConfiguration instead of an environment variable? I implemented ZEPPELIN_HOSTNAME as an environment variable to ease packaging in a docker image.

[Leemoonsoo]

This is where you can define your variable in ZeppelinConfiguration
https://github.com/apache/incubator-zeppelin/blob/master/zeppelin-zengine/src/main/java/org/apache/zeppelin/conf/ZeppelinConfiguration.java#L369

This is how to read the variable
https://github.com/apache/incubator-zeppelin/blob/master/zeppelin-server/src/main/java/org/apache/zeppelin/socket/NotebookServer.java#L290

[djoelz]

@jonbuffington what @Leemoonsoo said. Let me know if you need any help with this as I am responsable for the issue :D

[jonbuffington]

@djoelz Unfortunately, I am booked till this weekend. If you would like to make the changes, it would be appreciated.

[jonbuffington]

@Leemoonsoo Thanks for the pointers!

[djoelz]

@jonbuffington@Leemoonsoo don't worry guys I will make the fix. Will create a separate PR.
Thanks!

[felixcheung]

check for host.length > 1?

[Leemoonsoo]

The same issue is also being taken care of, by #235.

[jonbuffington]

I am closing this in favor of #235.