Reuse results from existing scan from Maven scan

[h3xstream]

Many use cases would benefit from running the scan separately from SonarQube.

Alternative Lifecycle :

1. Run mvn spotbugs:findbugs
2. Run mvn sonar:sonar [...]

During the sonar scan if findbugsXml.xml is present, the results from the existing report will be used.

Standalone would still be functional mvn sonar:sonar [...]

Issues that will benefit from this change:

• New configurations: Taint analysis flags #48 Ability to configure special flags
• Nested configuration #54 Ability to configure nested scanning
• java.lang.OutOfMemoryError  #117 Ability to configure the memory of the scanner
• Performance issue with sonar-findbugs-plugin(v3.4.3) #41 Ability to run older version of the scanner
• Apply fb-contrib rules to Unit Tests #18 (Will partiallay covered)
• Can not execute Findbugs 3.4.4 with Sonar-Scanner 3.0.0.702 (Android Gradle Project) #89 Better gradle integration (partial fix)
• Manually Importing findbugs-result.xml to sonar-cube #100 Feature request
• Ability to update quickly SpotBugs and fix faster bugs like Errors occurred during analysis using v3.5 #114, Missing Java 8 functional interfaces #123

[h3xstream]

Implemented https://github.com/SonarQubeCommunity/sonar-findbugs/releases/tag/3.6.0