Maint/master/parallelize

This PR introduces concurrent builds of mocks and debs in the jenkins workflows. Currently, all cows (usually about 10 per arch) are built serially, and the same goes for mocks. With this PR, cows/mocks are built in parallel, as many builders/executors as we have available. In repeated testing with facter 1.6.x, a "deb_all", e.g. a build of all cows, completed in approximately two minutes from rake invocation.