Supporting valid_systems = ['*:partition_name']

Hi,
We have multiple systems with the same partitions. Currently, in our tests, we need to specify a whole list for valid_systems, such as : 

``` 
self.valid_systems = ['cedar:cpu', 'graham:cpu', 'beluga:cpu', 'helios:cpu', 'niagara:cpu']
``` 

it would be great if it could be supported to have `['*:cpu']` in `valid_systems` 