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

[MichaelGoberling]

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

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/ACNA-3048