Support for rolling update functionality

[ishashchuk]

Addresses 51

Supports beta functionality on rolling updates.

rolling_update_policy block is stored in state file only(as opposed to applying it to InstanceManager object), and is applied at patch call only. That allows for flexibility of modifying it, as needed. If applied to InstanceManager, it can't be modified without re-creating the InstanceManager, but it can be modified on the patch calls.

I have also chose to have two separate arguments that are in conflict with each other for "fixed" and "percent" values. I think it's a little cleaner, since we can do appropriate validations on "Int"s and not have to convert values to appropriate types a bunch of times
As an alternative, I could consolidate them to a single argument of type string, and do regex parsing to figure out whether it's a percent value(contains '%') or fixed value(number only), but I seem to like the first approach a little better. Please, let me know what you think regarding this

[ghost]

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