Mesos-slave support for "node drain"

Given that multiple frameworks can be present on a machine at a time, and writing "node drain" for each possible framework is an intractable task, it would nice if the slave-master core had a means to tell frameworks that tasks were killed to drain a host. Or possibly that the slave was told to drain the host of all tasks (graceful shutdown, etc).

# drain current host
pkill -USR1 mesos-slave

This would make writing scripts for site-ops to do node maintenance much easier...