Remove the docker driver

The docker compute driver does not currently have CI running against
it.  In accordance with our plan to require this for all drivers, we
are now removing the driver from nova.

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

At this point, the driver should be set up in a stackforge repository
and have CI staged from there to prove its reliability before we could
consider bringing it back into Nova.

DocImpact
UpgradeImpact

Change-Id: I1c9bea2fdeebc4199c4f7d8fca4580a6fb7fed5b
Merge "Remove the docker driver"