Per-branch job validation

[dragos]

It'd be great to be able to configure the jobs that need to be run per branch. This is very useful for projects that support multiple Scala versions. For example, in the IDE we have:

• master -> supports scala 2.10.2-SNAPSHOT and 2.11.0-SNAPSHOT
• release/3.0.x -> supports 2.9.3 and 2.10.2-SNAPSHOT

Each Scala version has its own Jenkins job.

While a Jenkins job can check the mergebranch and "do nothing", it's not working well with some Jenkins plugins. For example, the JUnit results collector fails the build if there are no (new) test results.

[dragos]

For example, one "failed" job that should be a success: https://jenkins.scala-ide.org:8496/jenkins/job/pr-validator-master-trunk/677/console