Define machines deletion policies.

[p0lyn0mial]

The current implementation of MachineSet chooses the first machine from the list to delete. As this doesn't provide any guarantees of consistency regarding the order of the items. It is random. We would like to identify some strategies that would provide better approaches for deletion.

[p0lyn0mial]

/cc @rsdcastro

[mvladev]

If I would manually remove one Machine from a set of machines I would use the order bellow to determine which to remove:

1. Machine needs to be preferably removed - marked with annotation or some other means by human / cluster autoscaler (down-sizing scenario) / other controller
2. Health of the Machine - unhealthy machines first
3. If all else fails: select one at random

[rsdcastro]

This issue was moved to kubernetes-sigs/cluster-api#45