[ZEPPELIN-1695] chore: Set xml plugin phase to verify (minor)

[1ambda]

What is this PR for?

Changed the phase of xml validation plugin from validate to verify so that developers can use mvn test in submodule dirctories
(this PR doesn't affect on runtime application behaivor.)

[INFO] --- xml-maven-plugin:1.0.1:validate (default) @ zeppelin-markdown ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.643 s
[INFO] Finished at: 2016-11-28T17:53:47+09:00
[INFO] Final Memory: 17M/307M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.codehaus.mojo:xml-maven-plugin:1.0.1:validate (default) on project zeppelin-markdown: Failed to load schema with public ID null, system ID _tools/maven-4.0.0.xsd: /Users/lambda/github/apache-zeppelin/zeppelin-feature/markdown/_tools/maven-4.0.0.xsd (No such file or directory) -> [Help 1]
[ERROR]

What type of PR is it?

[Improvement]

Todos

What is the Jira issue?

ZEPPELIN-1695

How should this be tested?

$ cd markdown
$ mvn test

Screenshots (if appropriate)

Questions:

• Does the licenses files need update? - NO
• Is there breaking changes for older versions? - NO
• Does this needs documentation? - NO

[soralee]

Thanks @1ambda for the contribution 😄
I tested on Ubuntu and it works well as expected

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running org.apache.zeppelin.markdown.Markdown4jParserTest
SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/home/sora/.m2/repository/org/apache/zeppelin/zeppelin-interpreter/0.7.0-SNAPSHOT/zeppelin-interpreter-0.7.0-SNAPSHOT.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/home/sora/.m2/repository/org/slf4j/slf4j-log4j12/1.7.10/slf4j-log4j12-1.7.10.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.slf4j.impl.Log4jLoggerFactory]
log4j:WARN No appenders could be found for logger (org.apache.zeppelin.interpreter.Interpreter).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.162 sec - in org.apache.zeppelin.markdown.Markdown4jParserTest
Running org.apache.zeppelin.markdown.PegdownParserTest
Tests run: 13, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 1.888 sec - in org.apache.zeppelin.markdown.PegdownParserTest

Results :

Tests run: 14, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 7.581 s
[INFO] Finished at: 2016-11-28T17:59:03+09:00
[INFO] Final Memory: 20M/268M
[INFO] ------------------------------------------------------------------------

[bzz]

Looks great to me, thank you @1ambda

Merging to master if there is no further discussion.