provider/heroku: set app buildpacks from config

[bernerdschaefer]

Many apps deployed to Heroku require that multiple buildpacks be
configured in a particular order to operate correctly.

This updates the builtin Heroku provider's app resource to support
configuring buildpacks and the related documentation in the website.

Similar to config vars, externally set buildpacks will not be altered if
the config is not set.

Resolves #3698.

Acceptance test reuslts

make testacc TEST=./builtin/providers/heroku
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/24 11:41:14 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/heroku -v  -timeout 120m
=== RUN   TestProvider
--- PASS: TestProvider (0.00s)
=== RUN   TestProvider_impl
--- PASS: TestProvider_impl (0.00s)
=== RUN   TestAccHerokuAddon_Basic
--- PASS: TestAccHerokuAddon_Basic (5.44s)
=== RUN   TestAccHerokuAddon_noPlan
--- PASS: TestAccHerokuAddon_noPlan (7.30s)
=== RUN   TestAccHerokuApp_Basic
--- PASS: TestAccHerokuApp_Basic (3.45s)
=== RUN   TestAccHerokuApp_NameChange
--- PASS: TestAccHerokuApp_NameChange (7.06s)
=== RUN   TestAccHerokuApp_NukeVars
--- PASS: TestAccHerokuApp_NukeVars (6.81s)
=== RUN   TestAccHerokuApp_Buildpacks
--- PASS: TestAccHerokuApp_Buildpacks (9.65s)
=== RUN   TestAccHerokuApp_ExternallySetBuildpacks
--- PASS: TestAccHerokuApp_ExternallySetBuildpacks (6.45s)
=== RUN   TestAccHerokuApp_Organization
--- SKIP: TestAccHerokuApp_Organization (0.00s)
	resource_heroku_app_test.go:190: HEROKU_ORGANIZATION is not set; skipping test.
=== RUN   TestAccHerokuCert_Basic
--- PASS: TestAccHerokuCert_Basic (20.57s)
=== RUN   TestAccHerokuDomain_Basic
--- PASS: TestAccHerokuDomain_Basic (4.32s)
=== RUN   TestAccHerokuDrain_Basic
--- PASS: TestAccHerokuDrain_Basic (4.17s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/heroku	75.239s

[grubernaut]

LGTM, thanks for this!

[ghost]

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