driver: allow override of connection for secondary drivers

driver: allow override of connection for secondary drivers

When the test suite is running, we don't want to be triggering the
startup of daemons for the secondary drivers. Thus we must provide a way
to set a custom connection for the secondary drivers, to override the
default logic which opens a new connection.

This will also be useful for code where we have a whole set of separate
functions calls all needing the secret driver. Currently the connection
to the secret driver is opened & closed many times in quick
succession. This will allow us to pre-open a connection temporarily,
improving the performance of startup.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>