WIP: support variable substitution for cf apps

[o-orand]

TODO:

• illustrate usage in reference dataset
• add cf integration tests, related to #108
  • [ ] enable & load cf-app pipeline
  • [ ] generate appropriate docs/reference_dataset/config_repository/hello-world-root-depls/cf-apps-deployments/generic-app/enable-cf-app.yml
    • we can override values in this file

     cf_username: a-test-User
     cf_password: a-test-Password
     cf_organization: my-test-org
     cf_space: my-test-space
    

    • we can retrieve values from credhub (but I'm not sure our concourse has a credhub available)

     #find from /????/main name space
     cf_username: ((coa_cf_username)) 
     cf_password: ((coa_cf_password))
     cf_organization: ((coa_cf_generic_app_org))
     cf_space: ((coa_cf_generic_app_space))
    

  • [ ] tag tests requiring a run CF, to be able to skip its when no CF is available

Fixes #188.

Changes proposed in this pull-request:

• a file ending with vars.yml in application template dir is detected as file with variables to be injected into application manifest.