Support "droplet" argument for cf run-task

[risicle]

Very small request here.

What's the user value of this feature request?
Being able to nominate a droplet at task creation time allows the user to avoid potential race condition pitfalls relating to setting the app's "current" droplet.

(See also: what the user value was for adding this possibility to the underlying v3 api at all)

Who is the functionality for?
Likely people who are using cloudfoundry in a fairly automated fashion, with frequent rolling deployments and multiple droplets.

How often will this functionality be used by the user?
Well, we're looking at using it in our pipeline for database migrations so for us it would be run as part of every new deployment.

Who else is affected by the change?
Does this feature cause a breaking change? No.

Is your feature request related to a problem? Please describe.
I think I've probably already described this, but when you have quite a lot of things going on with your app, performing a mutation to it (by setting the "current droplet") is a bit of an unnerving thing to do if you don't necessarily know all the consequences.

Describe the solution you'd like
Probably a --droplet-guid flag for cf run-task.

Describe alternatives you've considered
We're currently using cf curl. It works. But this (really neat) possibility is very undiscoverable because it's not exposed in the cli.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/176721323

The labels on this github issue will be updated when the story is started.

[gururajsh]

Thank you for your submission. Have added this issue to our backlog. Will get back when we have an update.