This canges the output of the Docker endpoint to be a map so that it is
more consistent from single to multiple returns. It also refactors
internally how we handle both types of requests.
Without this PR the /docker API endpoint is broken completely so this
change in format has no effect anyways.
These changes are tested by the upcoming integration tests.
This is done by introducting the concept of "namespaces" of container
names. The aliases of a container are under this namespace. Namespace
names are of the form:
//<namespace>/<alias>
This allows us to (within cAdvisor) query all docker containers as
//docker regardless of whether this is a systemd or a non-systemd system.
This does break our ability to handle Docker aliases with the /container
endpoint. I think this is acceptable as our support there was not
consistent between system types.
