`aio rt api create` allows relpath without a leading slash

Currently users can run `aio rt api create basepath relpath ...` and it will succeed, producing an endpoint like https://adobeioruntime.net/apis/namespace/basepathrelpath

On the other hand, `wsk` requires that relpath begins with a `/`, to separate the basepath and relpath, and if users don't include it, the command produces an error:
```
error: Unable to parse api command arguments: 'relpath' must begin with '/'.
```

We should update `aio rt api create` to match this `wsk` behavior