Use Paginators for Stack SetInstance Validation

[flomotlik]

The python3.7 boto3 version doesn't support paginators for:

• list_stack_sets
• list_stack_set_instances

For now we've implemented our own paginator, but should switch when a newer boto is supported. Code:

paginator = cf.get_paginator('list_stack_sets')
[s['StackSetName'] for page in
    cf.get_paginator('list_stack_sets').paginate(Status='ACTIVE') for s in
    page['Summaries']]

paginator = cf.get_paginator('list_stack_instances')
di = {(i['Account'], (i['Region'])): i['Status'] for page in
      paginator.paginate(StackSetName=sn) for i in page['Summaries']}