The list of drivers suggested to the user may vary based on whether we
are installing a server or desktop image / source.
In the current implementation of the drivers controller, the value of
the source variant (e.g., server or desktop) is read early in the
initializer.
This is a problem because it happens before the client gets the
opportunity to tell us if we are installing a server or a desktop image.
If the source variant ever changes, we want to query again the list of
drivers to suggest.
Upon configuring the source, the drivers controller will query (again)
the list of drivers.
Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>