feat(generate_changelog.go): add changelog entries

[vdice]

For the Workflow repo itself (anticipated changelog) and auxiliary
repos such as workflow-cli and workflow-e2e (assumed to already be released)

The default/happy case test would be:

$ make build
$ # simulate last release
$ export WORKFLOW_RELEASE=v2.11.0 WORKFLOW_PREV_RELEASE=v2.10.0
$ helm repo add workflow https://charts.deis.com/workflow # if haven't done so already
$ helm fetch --untar workflow/workflow --version v2.10.0
$ # verify generated changelog shows workflow(-cli,-e2e) changes:
$ ./deisrel changelog global workflow/requirements.lock map.json

There haven't been any commits in deis/workflow since this last v2.11.0 release, but if there were, they would be seen in the following simulated upcoming release:

$ # simulate upcoming release
$ export WORKFLOW_RELEASE=v2.11.1 WORKFLOW_PREV_RELEASE=v2.11.0
$ helm fetch --untar workflow/workflow --version v2.11.0
$ ./deisrel changelog global workflow/requirements.lock map.json

Ref deis/workflow#722

[sgoings]

Do we still use this? I thought we're using upstream k8s' registry-proxy now...

[vdice]

Ah good point -- not quite merged yet (deis/workflow#734) but should be by end of this milestone. Looks like just one last change technically not released (deis/registry-proxy@v1.1.1...master) but I assume we're fine with dropping that? If so, I'll remove this line.

[vdice]

removed

[mboersma]

I tested this as described in the "simulate last release" section and see appropriate CHANGELOG entries for workflow-cli and friends.