have downloadIstioCandidate.sh ask github for the latest (#1977)
have downloadIstioCandidate.sh ask github for the latest (#1977)

Automatic merge from submit-queue.

have downloadIstioCandidate.sh ask github for the latest

This change allows downloadIstioCandidate.sh to automatically ask github for the latest version.  It seems to work on Mac and Linux and I've preserved the behavior to allow ISTIO_VERSION to override the version.  This file isn't to be confused with ../downloadIstio.sh that's used to grab the latest stable.

The main tradeoff I can think of (besides if I've unknowlying added an inappropriate dependency) is that if we have a release like 0.3.0 and then do a new release for a patch update (e.g., a hypothetical 0.2.15) then github might report that 0.2.15 is the latest.  I'm not sure I have a good solution for this.  Even if I have the script take the max(github-latest, ISTIO_VERSION-in-file) then this won't be ideal either if the version in the script is stale, and my eventual hope/goal would be to get rid of the baked-in version (and the need to update it).

Thoughts on whether this is a worthwhile or achievable goal?

```release-note
NONE
```
istio/istio,istio/istio,geeknoid/istio,smawson/istio,smawson/istio,geeknoid/istio,smawson/istio,istio/istio,geeknoid/istio,smawson/istio,geeknoid/istio,geeknoid/istio,istio/istio,smawson/istio,geeknoid/istio,frankbu/istio,frankbu/istio,frankbu/istio,frankbu/istio,smawson/istio,frankbu/istio,frankbu/istio,istio/istio