If castellan is used, it requires one of the following context types to
be provided during init: KeystonePassword, KeystoneToken or
RequestContext [1]
Also it makes sense to be consistent across projects and with
oslo_context regarding context class naming.
[1] 86712360f3/castellan/key_manager/barbican_key_manager.py (L208-L210)
Change-Id: Ic08ad89b4e07e0fe8a80b3e5db6d50276aafff82