Spot instances: support BlockDurationMinutes

[domenkozar]

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/spot-requests.html#fixed-duration-spot-instances

You can specify a duration of 1, 2, 3, 4, 5, or 6 hours. The price that you pay depends on the specified duration. To view the current prices for a 1 hour duration or a 6 hour duration, see Spot Instance Prices.

We'll have to use boto3 to be able to pass this parameter.