add src-neutron and src-largeops jobs for libraries

This brings in tempest-dsvm-neutron-src and tempest-dsvm-largeops-src
jobs for libraries to use to test that their next release will not
break OpenStack.

It also provides lengthly comments about these jobs and their intended
purpose for future generations.

Change-Id: I88438769d1ed269d271804bda354f0bca8b6642a