Cleanup when sdn-plugin provider has gone away

[chuckbutler]

In the current code in master, the docker daemon is left in a broken
state if the SDN provider ever goes away.

• renames the private method _reconfigure_docker_for_sdn to a much
  clearer indication of whats happening
• Adds state detection to determine when we should stop workloads and
  reconfigure the daemon
• Leverages the new methods added to the DockerOptsManager in
  charms.docker 1.11.0 for popping flags out of the dict.

[wwwtyro]

What if bip isn't in opts but mtu is? Seems like we'd fail to remove mtu -- is that acceptable?

[chuckbutler]

hmmm, seems odd that you would have an interface MTU configuration without BIP. But i concede the point, that its possible a prior execution could have unintended side effects with checking both inline like this instead of independent checks.

[mbruzek]

For how I understand the DockerOpts object work, we need to pop the key/value off before running. This change looks good to me. +1