Cleanup Maven warnings & unify plugin management

[WARNING] Some problems were encountered while building the effective model for org.apache.zeppelin:zeppelin-jupyter-interpreter-shaded:jar:0.9.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-shade-plugin is missing. @ line 61, column 15
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.apache.zeppelin:spark-interpreter:jar:0.9.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin com.googlecode.maven-download-plugin:download-maven-plugin @ line 411, column 15
[WARNING] 'build.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.maven.plugins:maven-resources-plugin @ line 591, column 15
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