Add a skip condition to all tests slower than 100ms.
This change was made using `gotestsum tool slowest` with data from the
last 3 CI runs of master.
See https://github.com/gotestyourself/gotestsum#finding-and-skipping-slow-tests
With this change:
```
$ time go test -count=1 -short ./agent
ok github.com/hashicorp/consul/agent 0.743s
real 0m4.791s
$ time go test -count=1 -short ./agent/consul
ok github.com/hashicorp/consul/agent/consul 4.229s
real 0m8.769s
```
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
This way we can avoid unnecessary panics which cause other tests not to run.
This doesn't remove all the possibilities for panics causing other tests not to run, it just fixes the TestAgent
Ronald