providers/heroku: use a helper for common app post-create tasks

[danp]

Buildpacks were being configured post-create for non-org apps but not
for org apps. Consolidate post-create tasks into a helper used by both
paths.

[catsby]

LGTM!

[github.com/hashicorp/terraform][pr-13990](4)$ make testacc TEST=./builtin/providers/heroku TESTARGS="-run=TestAccHeroku"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/26 17:00:33 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/heroku -v -run=TestAccHeroku -timeout 120m
=== RUN   TestAccHerokuAddon_Basic
--- PASS: TestAccHerokuAddon_Basic (4.31s)
=== RUN   TestAccHerokuAddon_noPlan
--- PASS: TestAccHerokuAddon_noPlan (7.39s)
=== RUN   TestAccHerokuApp_Basic
--- PASS: TestAccHerokuApp_Basic (2.78s)
=== RUN   TestAccHerokuApp_NameChange
--- PASS: TestAccHerokuApp_NameChange (5.26s)
=== RUN   TestAccHerokuApp_NukeVars
--- PASS: TestAccHerokuApp_NukeVars (6.16s)
=== RUN   TestAccHerokuApp_Buildpacks
--- PASS: TestAccHerokuApp_Buildpacks (7.14s)
=== RUN   TestAccHerokuApp_ExternallySetBuildpacks
--- PASS: TestAccHerokuApp_ExternallySetBuildpacks (4.47s)
=== RUN   TestAccHerokuApp_Organization
--- PASS: TestAccHerokuApp_Organization (3.50s)
=== RUN   TestAccHerokuApp_Space
--- SKIP: TestAccHerokuApp_Space (0.00s)
        resource_heroku_app_test.go:220: HEROKU_SPACE is not set; skipping test.
=== RUN   TestAccHerokuCert_Basic
--- PASS: TestAccHerokuCert_Basic (18.61s)
=== RUN   TestAccHerokuDomain_Basic
--- PASS: TestAccHerokuDomain_Basic (3.37s)
=== RUN   TestAccHerokuDrain_Basic
--- PASS: TestAccHerokuDrain_Basic (3.46s)
=== RUN   TestAccHerokuSpace_Basic
--- FAIL: TestAccHerokuSpace_Basic (0.17s)
        testing.go:280: Step 0 error: Error applying: 1 error(s) occurred:

                * heroku_space.foobar: 1 error(s) occurred:

                * heroku_space.foobar: Post https://api.heroku.com/spaces: Space management is not enabled for this team.
FAIL
exit status 1
FAIL    github.com/hashicorp/terraform/builtin/providers/heroku 66.649s

I don't think the last failure is related 😄

[catsby]

Also, "hi Dan 👋"

[ghost]

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