Fix addBuildsetForSourceStampsWithDefaults
 - project is a mandatory parameters of findSourceStampId. So set it to '' by default.
   addBuildsetForChanges does the same thing when a codebase has no changes. (See schedulers/base.py line 249)
Merge delanne/buildbot:FixPeriodicScheduler (PR #1326)
Add implementation for a DockerLatentBuildSlave
Add an entrypoint for the DockerLatentBuildSlave
Build the image if not found and the dockerfile is given
Add tests for the DockerLatentBuildSlave
Add documentation for the latent BuildSlave
Small fixes
defer the stop_instance to a thread, don't wait if fast.
Fix the comment about container isolation in the doc. (Thanks @sa2ajj)
Prefix the thread methods with 'thd_'
Don't make the test dependant on private methods