System restart support

__DO NOT MERGE -- must occur in unison with several other repos__

These changes are part of the broader system restart support. On a high level, the following changes were made to the launch plans:
- Added a ``restart.conf`` top-level plan file which is used to restart an existing (but terminated) launch. You must use the same run name, but the plan itself may be changed and regenerated.
- Introduced awareness of when system launch is happening into the Process Dispatcher. At the beginning of the boot (``zookeeper-bootstrap`` service), a *system boot* flag is set in ZooKeeper. The PD sees this during initialization. At the very end of the launch the ``pd-finalize`` service messages the PD via ceictl to indicate the end of system boot. This is used to hold off restarting any processes from prior launches until the system is fully booted.
- Made ``dtrs-bootstrap`` and ``pd-bootstrap`` services cleanly support launching with preexisting objects in the store. Objects are updated as needed.