Run checkstyle plugin during package scope

1. Enable checkstyle verification during package phase. Currently it validates and writes result to console and target/checkstyle-result.xml files. 
2. Verification does not fail the build. Once all issues are fixed we can set `<failOnViolation>false</failOnViolation>` to true
3. Checkstyle rules are based on Falcon's rules, but with some modifications. Main modifications are - javadoc not required for package scope, allow package scope variables, local final not needed everywhere etc.
4. Some checks disabled for test code
Falcon is not setting checkstyle's parent as well -
```
 20 <project xmlns="http://maven.apache.org/POM/4.0.0"
 21          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 22          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 23     <modelVersion>4.0.0</modelVersion>
 24     <groupId>org.apache.falcon</groupId>
 25     <artifactId>checkstyle</artifactId>
 26     <version>0.6-incubating-SNAPSHOT</version>
 27     <name>Checkstyle</name>
 28 </project>
```