[Addon]: VelaUX ingress expose class name parameter

[oanasc]

Description of your changes

Fixes ##739

How has this code been tested?

Tested by enabling the addon on a cluster with multiple nginx's and confirmed the ingress was correctly created and UX running
vela addon enable velaux/ domain=<mydomain> gatewayDriver=nginx class=<myclassname>

Checklist

I have:

• Title of the PR starts with type (e.g. [Addon] , [example] or [Doc]).
• Updated/Added any relevant documentation and examples.
• New addon should be put in experimental.
• Update addon should modify the version in metadata.yaml to generate a new version.

Verified Addon promotion rules

If this pr wants to promote an experimental addon to verified, you must check whether meet these conditions too:

• This addon must be tested by addon's e2e-test to guarantee this addon can be enabled successfully.
• This addon must have some basic but necessary information.
  • An accessible icon url and source url defined in addon's metadata.yaml.
  • A detail introduction include a basic example about how to use and what's the benefit of this addon in README.md.
  • Also provide an introduction in KubeVela documentation.
  • It's more likely to be accepted if useful examples provided in example dir.