This change adds a section to the developer guidelines about using the
key manager within the sahara codebase. It also adds some minor cleanup
to the sahara_key_manager docstrings.
* add a section to the development guidelines about the key manager
* add an anchor in the advanced configuration guide for the key manager
* improve the docstrings in the sahara.service.castellan.utils module
* cleanup the docstrings in the
sahara.service.castellan.sahara_key_manager module
Change-Id: Ied9278113deab5af0bbd80d229810e0636bb2b72
Partial-Implements: blueprint improved-secret-storage
3625f307f9