feat: Add variable to pin showroom deployment to a specific user/namespace in non-workshop labs

[prakhar1985]

Problem

When a lab creates multiple users (e.g. user1, user2) but is NOT a multi-user workshop, the ocp4_workload_showroom role has no way to pin showroom to a specific user. The branching logic is entirely driven by whether agnosticd_user_data.users is present — if it is, the role loops over ALL users and creates one showroom namespace per user.

This creates friction for labs that:

• Create 2+ users for application purposes (e.g. admin + regular user)
• Only want a single showroom instance scoped to one of those users
• Are NOT multi-user workshops

Current Workaround

Don't register users under agnosticd_user_data.users and override the namespace directly:

ocp4_workload_showroom_namespace: "user1"

This is fragile — if another workload registers users under agnosticd_user_data.users, the showroom role switches to multi-user mode unexpectedly.

Requested Change

Add a variable such as ocp4_workload_showroom_target_user (or similar) that, when set, forces the single-user path and uses the specified username to derive the namespace — independent of agnosticd_user_data.users.

ocp4_workload_showroom_target_user: user1

This allows catalog authors to explicitly control which user showroom is deployed for, without having to avoid registering other users in agnosticd_user_data.