Configure static code analysis as part of code scan

Currently OpenWhisk build run a [scalafmt][1] check to enforce a consistent code style. We should look into configuring some static code analysis check also to enforce certain implementation rules like using `Seq.empty` instead of `Seq()`

We have following options

1. [scapegoat][2] - Scala compiler plugin for static code analysis
- [rules][9]
- Has some useful rules
2. [scalafix][3] - rewrite and linting tool for Scala
- [rules][8]
- From Scalacenter
3. [scalastyle][4]
- Gradle Support
- Most of [the rules][5] are around formatting
4. [linter][6]

Linter docs link to [many other][7] similar tools

Most of the tools do not have direct Gradle support :(

[1]: https://scalameta.org/scalafmt/
[2]: https://github.com/sksamuel/scapegoat
[3]: https://scalacenter.github.io/scalafix/
[4]: http://www.scalastyle.org/
[5]: http://www.scalastyle.org/rules-1.0.0.html
[7]: https://github.com/HairyFotr/linter#future-work
[6]: https://github.com/HairyFotr/linter
[8]: https://scalacenter.github.io/scalafix/docs/users/rules
[9]: https://github.com/sksamuel/scapegoat#inspections

Start with the existing scalafmt check in the Gradle build and review how code scanning is configured. Compare scapegoat, scalafix, scalastyle, and linter for Gradle integration and useful implementation rules. Done means one suitable static analysis tool is configured as a code-scan check with agreed rules, including the Seq.empty preference.

Written by the indexing model from the issue text.