Refactor secret_store for consistency

Mostly re-arranged the code a bit, so that for...else... Statements are
used in a more consistent way. On the other, the exception
SecretStorePluginsNotConfigured was introduced to more properly reflect
the occations where the plugins have not been configured correctly.

Change-Id: I84693a65c6e6b44cb81973590ff8c62f28c0b068
Merge "Refactor secret_store for consistency"