change: do not use default ingressClass annotation and fix traefik uninstall warning

[iwilltry42]

Ref: #649

Additional info:

• before this change, acorn uninstall yielded an error Error: an empty namespace may not be set when a resource name is provided because the Service wasn't assigned to a namespace
• Traefik will only be installed, if no ingressClassName was specified via flag
• this PR also adds the success message for the Post-install checks

Signed-off-by: Thorsten Klein [email protected]