Check if the pool id have already been attached

Sometime subscription-manager returns an error but have subscribed to
the pool ID. Add a check to ensure that the pool ID have been attached
and not let the subscription task fail.
@Ichimonji10 if I name it as a constant then I get pylint variable in function should be lowercase. Also I have not declared it as a module constant because it is very specific to the subscribe task and create a single use var because that string in the if condition will not be good to read