WebApps_CreateOrUpdate : name should have regexp

Root issue: https://github.com/Azure/azure-sdk-for-python/issues/3673

> create_or_update needs to validate if provided name is valid or not. E.g. underscore is not allowed and you can see it via the portal.
>
> This method sendsa request, response contains 400: Bad request with no details. Hard to guess the issue.

This method should have a regexp to check that name matches what is correct for the service

Find the WebApps_CreateOrUpdate operation definition in this repository and read the linked issue for the service's name requirements. Add validation for names that do not match the required pattern, including underscores, and confirm that valid names remain accepted before requests are sent.

Written by the indexing model from the issue text.