Apply role assignment to Service Principal only once per sub

[Phydeauxman]

Benefit/Result/Outcome
Remove duplicate attempts to assign the same role to the same scope

Description
Current Service Principal code attempts to assign the Contributor role to each tier sub. When deploying to a number of unique subs that is less than the number of tiers (like deploying all tiers to a single sub)...the second and subsequent assignment attempt results in a return message saying that the Service Principal has already been assigned that role to that sub.

Acceptance Criteria

• Assignment is attempted only once per unique sub ID