Currently when data is imported via `consul kv import` it overwrites
keys under the root key. Since `consul kv export` can retrieve data for
the given prefix, i.e. part of the KV tree, importing it under root may
be not what users want.
To mirror prefix behavior from export this PR adds prefix feature to the
import command that adds prefix to all keys that are imported.
Also update the Docs and fixup the HTTP API to return proper errors when someone attempts to use Namespaces with an OSS agent.
Add Namespace HTTP API docs
Make all API endpoints disallow unknown fields
