Handle plugin containers stopping

[lukemarsden]

When a plugin container dies or is removed, it should be removed from the list of plugins that endpoints iterate over.

However if a plugin container has a restart policy, it should be able to reconnect and re-register as a plugin.

[lukemarsden]

Currently lack of plugin deregister event means that many dead plugins are building up during test runs:

[vagrant@node1 ~]$ sudo docker info                                                                              
Containers: 1
Images: 82
Plugins: 7
Storage Driver: devicemapper

(https://github.com/ClusterHQ/powerstrip-flocker/blob/clean-volume-plugin/powerstripflocker/test/test_acceptance.py, #17 etc)