Support injection of database configuration in SqlBase

Problem/Motivation

Migrate's SqlBase class depends on global state - a hard-coded 'migrate' database connection which must be defined elsewhere (settings.php or explicit addConnectionInfo). This limits the flexibility of the class (for one thing, it means all migrations can only use the one connection, you can't have migrations from multiple databases), and could conflict with environments that define a 'migrate' connection for other reasons.

Proposed resolution

The class already supports a 'target' configuration key for the connection target - add 'key' for the connection key, and 'database' for a database info array to define the connection.

Remaining tasks

Submit the patch.

User interface changes

None.

API changes

Configuration additions:

• key - Connection key string, if omitted default to 'migrate' as today.
• database - A database information array as one would find in settings.php. If present, passed to Database::addConnectionInfo() to define the connection.