Make the 'count' property of the 'retry' policy optional

In some cases one wants to have a loop in a workflow similar to a regular 'while' loop in regular programming languages. The easiest way now to do so is to use 'retry' policy. However it requires the 'count' property to be a positive integer so that it limits a number of attempts of the policy. I think we can soften that requirement and make 'count' optional and if it's omitted we can require users to define either 'break-on' or 'continue-on', or both.