Add docker driver removal warning

This adds a log message during the docker driver host init phase to
warn administrators that the driver currently does not meet the
requirements set out in:

https://wiki.openstack.org/wiki/HypervisorSupportMatrix/DeprecationPlan

Per the timeline above, this warning is being added in the Icehouse-2
milestone phase as a warning to anyone using it in the event that
the requirements are not met. While progress is being made towards
meeting the requirements, there is no visible indication that the
goal will be met. If it is, this is an easy revert.

Change-Id: I39318ed6544c77b4aa0f4614c16bc14a93734bf6
Merge "Add docker driver removal warning"