Web UI integration tests: Compute range sizes to avoid overlaps
    
    Heavily inspired by code from xmlrpc tests.
    
    To obtain ranges, this patch also adds method to execute FreeIPA command through Web UI.
    It uses Web UI instead of ipalib so it doesn't need to care about authentication on a test-runner machine.
    
    https://fedorahosted.org/freeipa/ticket/3744