Add living user documentation generated from tests

[o-orand]

As a paas-template author, in order to use COA features, I need an accurate, up-to-date documentation with examples of syntax I can copy/paste.

The living documentation BDD practice may be helpful here. See https://prezi.com/hzxuat_opltx/turning-tests-into-documentation/ and related video for background (similar in Specs by Example methodology).

The current Rspec tests are primarily targetted towards COA contributors.

• Rspec test results may be published as documentation to better assess how useful they can be to paas-template authors.
• Maybe we can use RSpec HTML Reporter to have nicer reports

Specific tests/documentation targetted towards COA users may leverage specific bdd tooling/framework such as cucumbern which may ease consumption by COA users.

Note: It could be nice to finalize, code climate setup, to have code coverage. This may deserve its own issue.

[gberche-orange]

@poblin-orange 's suggestion to inspire on format from https://github.com/deepmind/kapitan

[o-orand]

We could also list all secrets required by generated pipelines to help users to setup concourse vars_files or credhub

[o-orand]

closed by PR #149