clean workspace after build

[Karthik Balu]

Filling up disk space, to be able to clear workspace after build or limit the stored build results

[Naoki Takezoe]

Currently, build results are kept by up to 20 for each projects. This number is defined at:

gitbucket-ci-plugin/src/main/scala/io/github/gitbucket/ci/manager/BuildManager.scala

Line 9 in e8b09ed

 val MaxBuildHistoryPerProject = 20 

It should be configureble in the admin console.