Import scripts verbatim from jenkin-scripts
Towards travis-style CI

Move script to the repo it acts on, out of scala/jenkins-scripts

Had to make some changes:
 - scala checkout is now in root ($WORKSPACE)
 - credentials are actually in homedir (sonatype/private-repo)
 - inline pr-scala-common
 - don't run tests for actors-migration due to timeout (TODO: revert?)
Merge pull request #4225 from adriaanm/jobs

Refactor towards Travis CI-style: import jenkins-scripts jobs