[FIXED JENKINS-17576] [FIXED JENKINS-16870] support conditional buildstep plugin

This PR fixes two issues: 

https://issues.jenkins-ci.org/browse/JENKINS-16870
https://issues.jenkins-ci.org/browse/JENKINS-17576

Phases can now be wrapped within a condition and therefore very flexible configurations are supported.
e.g. a phase can be skipped by buildparameters or if a file exists - basically every condition supported by the conditional buildstep can be used.