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
This has an explcit unit test already which somehow passes at least some of the time. I suspect it passes because under some conditions the actual KV delete fails and returns non-zero as well as printing the warning which is what is being checked for in the test.
For some reason despite working for quite some time like this, I now have a branch in which this test fails consistently. It may be a timing/env issue where another process running an agent causes the delete to be successful so the command returns a 0 by chance. Either way this is clearly wrong and fixing it stops the test being flaky in my branch.
