[JENKINS-8597] Deal with matrix builds better
MatrixProject was not found during init of plugin.

getAllItems() did not return MatrixProject as it is a ItemGroup
Fixed bad startup sequence

PrioritySorter was created before all jobs was loaded, 
this created a race condition when checking legacy.

Also making sure vital parts are running as SYSTEM user.
Merge pull request #9 from emsa23/UpdateStartupSequence

Deal with MatrixBuilds and Update startup sequence