acs: expose scale command

[yugangw-msft]

Notes:

1. Expose scale command which updates the private agent count, a common scenario.
   2 Hide the update, as there are nothing else people can tweak. The tagging coming with the generic update can be accomplished by az resource tag.

[yugangw-msft]

//cc @sauryadas @derekbekoe @tjprescott

[derekbekoe]

LGTM

[tjprescott]

I don't understand the reason for this change. If the previous version let you update the agent-count and update the tags, why would we remove the tagging ability and direct people to use the more obscure az resource tag just we can rename the command from update to scale (which then necessitates the awkward name of new-agent-count that was already implied in the update command)?

To me it would make more sense to keep update, rename 'agent-count' to 'instances' (which I think is clearer) and expose --tag.

[sauryadas]

@tjprescott the scale command is more intuitive. With update, it can mean updating VM Sizes, no of masters, location , etc. which we want to avoid. You can both scale in and out with this command which is why we call it out with new-agent-count to point out that this is your new cluster size. Open to other suggestions.

[yugangw-msft]

@tjprescott, per discussion, opened #1149 to track the work of getting the generic update command back when conditions are met. So far, we agree with @sauryadas, that we only need scale