'Get capacity of the different slo/ workload combinations.
:param array: the array serial number
:param srp: the storage resource srp
:param slo: the service level
:param workload: the workload
:returns: remaining_capacity -- string, or None'

def get_headroom_capacity(self, array, srp, slo, workload):

    params = {'srp': srp, 'slo': slo, 'workloadtype': workload}
    try:
        headroom = self.get_resource(array, 'wlp', 'headroom', params=params)
        remaining_capacity = headroom['headroom'][0]['headroomCapacity']
    except (KeyError, TypeError):
        remaining_capacity = None
    return remaining_capacity
