Add configurable build message

Having a hard-coded HTML description is pretty inconvenient when you don't want to allow raw HTML descriptions, or if you'd rather have an altogether different build description message. This pull request adds a customisable template for the build message.
From my understanding of how the config files are read the configure will be called, and it doesn't matter what the default strings were, they will all be updated.  The global.jelly file would need to have 'default='${descriptor.buildDescTemplate}''  I am going to update the defaults so there isn't any more confusion on this.