allow admins to force app deployment on a specific host

Currently you ask YARN to get slots on a host and it finds a slot on that machine -or, if unavailable or there is no room, on a host nearby as far as the topology is concerned.

People with admin rights should have the option to deploy a process on a specific host and have it run there even if there are no free slots -and to fail if the machine is not available. This would let you deploy admin-specific process across a cluster.