Busybox image not configurable

[joaquinfll]

Bug description
The location where busybox is downloaded is not configurable.

Expected behavior
Busybox image should be configurable.

Steps to reproduce the bug
Install Istio with Helm
Prometheus will fail because busybox cannot be downloaded.
Version (include the output of istioctl version --remote and kubectl version)
1.1.3

How was Istio installed?
With Helm
Environment where bug was observed (cloud vendor, OS, etc)

Additionally, please consider attaching a cluster state archive by attaching
the dump file to this issue.

In a restricted environment an internal docker registry is commonly used, having deployments without configurable image values make it not possible to deploy easily.

[sdake]

Depending on how tidy the change is, this should be considered for backport to the 1.1 series. Not being able to deploy in a firewalled environment is a problem.

@duderino agree?

Cheers
-steve

[linsun]

agreed this should be configurable, most of our images are already configurable, think we missed this one.

[bianpengyuan]

Init container has been removed. so configurability is no longer needed.