Move machine_types tuple to Node class
Add job_stats API to NodeController

For queries like:
/nodes/job_stats/ -> job stats for all nodes
/nodes/job_stats?machine_type=plana -> job stats for all planas
Make keys in job_nodes table primary
Add set_targets command
Make more Node columns nullable

It seems important to be able to create a Node via:
Node('planaXX.front.sepia.ceph.com')
Optimize job_stats queries

Instead of taking 20+ seconds, it now takes 0.13
Improve set_targets command

When it's rerun, it will skip jobs it's processed before. Also, it will
make sure nodes have their machine_type field set.
Order results of job_stats query by (fail + dead)