Forward slashes are not allowed in broker virtual host in profile setup

[sphuber]

The new broker configuration parameters that have been added in v1.4.0 have an optional broker virtual host field. Currently, names containing a forward slash fail validation, because the validator is based on a hostname validator, which cannot contain forward slashes. However, virtual hosts typically start with a forward slash, so users may be tempted to pass it but will be presented with a validation error. The help string message is misleading because it says that forward slashes need to be encoded.

The code that forms the URL based on the broker configuration of the profile will take care of adding a leading forward slash in case of a virtual host and it will encode the entire path. So at the very least, we should adapt the help string to say that the leading slash is not necessary and remove the comment about encoding. Ideally, any leading slashes are stripped before passing it to the validation.