Support terraform-provider-cf

[gberche-orange]

Add support for migrating to https://github.com/mevansam/terraform-provider-cf

This is a follow up of #23

[gberche-orange]

@o-orand terraform-provider-cf version 0.9.8 was just released with import support. This sholuld help us kick off the migration. Can you please include the provider in the docker image used by COA ?

[gberche-orange]

Summary of related design session on Sep 12th:

• a big bang to switch from orange TF provider CF to mevansam provider CF is risky and hard as: *this requires to synchronize coa, paas-templates (TF specs updates + TF state migration scripts in all root deployments) and paas-secrets TF (TF specs overrides)
• a gradual iterative approach is preferrable
  • coexistence of orange TF provider CF and mevansam provider CF in the same TF state is an expensive step, as it requires
    • tf provider renaming
    • tf specs resource renaming and update of cross references
    • tf state tweaks, see https://github.com/mevansam/terraform-provider-cf/pull/85#issuecomment-395998798
  • a progressive switch root deployment by root deployment seems preferable, with steps such as:
    • cloudflare-deps
    • coab-deps: migrate existing resources (brokers) + experiment deploy new resources (cf_app)
    • master-deps: migrate existing resources (spaces, orgs, brokers, service plan visibility ...)

Two approaches were considered for the "progressive switch root deployment by root deployment":

• invest and prioritize into docker image override per root deployment #189
• add transient specific coa support to switch between TF cloudfoundry providers (mevansam/ vs orange-cloudfoundry/ per root deployment
  • include binaries for both providers in the docker image, with a default one being active
  • tweak coa tf task to select which provider to load at runtime depending on new environment variables: CURRENT_DEPL and USE_ORANGE_TF_PROVIDER_ON_DEPLS
  • introduce new global concourse pipeline variable that can be progressively turned on in paas-secrets ("USE_ORANGE_TF_PROVIDER_ON_DEPLS=coab-depls master-depls micro-depls ops-depls kubo-deps"
  • potentially each release would provide scripts to edit this variable as new root deployments switch to the new provider

[gberche-orange]

the USE_ORANGE_TF_PROVIDER_ON_DEPLS flag should be set as part of paas-template and not paas-secret

[o-orand]

On hold => spike in progress to use a dedicated terraform boshrelease