Remove config parameter from secret_store.py interface

When the barbican.plugins.secret_store.py interface was added, the
existing HSM-style plugins were placed into an adapter module called
store_crypto.py. Since this module, unlike other secret-store
technologies such as Dogtag or KMIP requires access to lower level data
store resources such as KEKDatum, these resources were added to a
'context' parameter. Unfortunately all secret_store.py implementors had
to inherit this contract. Hence this CR removes that constraint, and
starts the process of moving towards singleton-based repository
instances.

Note as well that the secret_store.py module now has 100% unit test
coverage.

Change-Id: I7094beb4d593a6ac7a9de3cb818ac455f5c6ec1b
Merge "Remove config parameter from secret_store.py interface"