Canary with Flagger

[vfarcic]

This PR adds the support for canary deployments with Flagger and Istio to the Go build pack. If accepted, I can make a separate PR with the same set of additions to other build packs. All the changes are inside if statements so merging this PR will not affect users unless they change the canary.enabled value to true. I also used this opportunity to add the support for HPA (also not used without changing.

This PR is useful by itself. Nevertheless, there are some pending tasks that could be done in separate PRs.

1. Update the docs, write an article, record a video, or create some other way to explain how canary deployments work.
2. Apply the same set of changes to other packs.
3. Create a mechanism to auto-configure Istio Gateway in a similar way as today we're auto-configuring nginx Ingress.
4. Fail the environment pipeline if canary deployment fails.
5. If canary deployment fails, update the environment repo to reflect the change done by Flagger rolling back to the previous release.
6. Prevent pipelines from running in parallel as a way to prevent overlapping canary deployments.

If this PR is merged, I can come back with PRs for 1) and 2) fairly quickly.

[jstrachan]

for 6. we could use something like this idea jenkins-x/jx#5471

[jstrachan]

@vfarcic this is awesome - well played!

Minor knit - we maybe want to make the canary provider istio configurable via the values.yaml so folks could try linkerd/gloo too?

[jstrachan]

we can look at supporting gloo/linkerd too later on though; lets not slow down this change happening

[jstrachan]

/lgtm