[FLOC-4480, FLOC-4483] Fix docker systemd dropin configuration files and add docker plugin ID for Docker 1.12

[wallnerryan]

In order for one of these to pass, its needs the other. Creating one branch for both.

See:

• https://clusterhq.atlassian.net/browse/FLOC-4480
• https://clusterhq.atlassian.net/browse/FLOC-4483

NOTE- this only fixes acceptance testing for Docker >= 1.12. This will likely break acceptance tests for older versions of docker < 1.12. If we plan on fixing that we should create a seperate issue that allows us to flip flop these type of configurations based on the docker version we want to test.

[wallrj]

I think we should keep '--tlsverify. It ensures that docker daemon only accepts TLS connections from clients with a certificates signed by the Docker CA.

  --tls                                    Use TLS; implied by --tlsverify
  --tlscacert=~/.docker/ca.pem             Trust certs signed only by this CA
  --tlscert=~/.docker/cert.pem             Path to TLS certificate file
  --tlskey=~/.docker/key.pem               Path to TLS key file
  --tlsverify                              Use TLS and verify the remote

[wallrj]

Thanks @wallnerryan

Looks good.

• A couple of the Rackspace acceptance test nodes got killed before the tests finished....I've restarted those.
• I'd like to put back the --tlsverify flag for security.

I'll do that and kick off the tests again and then merge.