Implement missing job dsl pieces and use of docker slaves in seed.job

[buep]

When we have access to use a docker image as build slave, see https://github.com/Praqma/praqma-jenkins/issues/70 we should finish our seed.groovydsl doing:

• make the jobs use the new docker container slave with Maven
• enable pretested integration in our job
• the beta job should be able to run without a commit or replace back to real version number. The result should be that we are able to run both beta release job and real release job independently, so take of they both commit to git and push. It's a bit tricky.

[buep]

Part of https://github.com/Praqma/pretested-integration-plugin/issues/113