pluggable credentials storage

Split off from #1998078: support different configuration on different environments:

> b. we want the login credentials to the live service to NOT be in the codebase in Features, as not all developers with access to the codebase are to be granted access to use the live service's data

This is going to mean adding a concept of a 'credentials storage plugin': for each connection, you choose where the username & password are stored.

The plugins to start with are:

* connection configuration storage. This works the same as the current system. Username and password are in the serialized configuration array, and get baked into features.
* variable storage. The credentials are stored in a site variable. This means they're *not* exported to features.