Proposal: ability to update Target properties based on target status

[Haishi2016]

Requirement: monitor a group of Targets and when certain state changes match the designated criteria, update the properties of selected targets based on a pre-defined rule. For example, a user may need a group of Target to be running at the same time. And when some of the Target fail, add another target to the group so that the number of running targets can be brought back to quota.

Proposed changes:

1. Support a new "target group" component type, which spec specifies: 1) target selector to select Target to be considered; 2) rules to trigger target updates. We can start with number of running targets; 3) defines target selector to select additional Target to be considered; 4) defines rules how the target properties are patched
2. Support a new "target group provider" that is able to handle above "target group" component.