Ability to specify docker host

docker command line support various formats for
specifying the host including alternate locations
for unix sockets and option for setting remote
host and port as well. We should be able to do
the same in the nova docker driver

DocImpact

Change-Id: I83836bce722a06afa5cf6bfd8c0c5866f8d9a9b5
Merge "Ability to specify docker host"