Add an option for the buildChooser

Depending on what plugins are installed, the user may want
to specify a different "chooser strategy" via the buildChooser
field. Add an option for doing so.

Change-Id: Iba59f8c0a8446c88f64ed8b70b3639a89d9b9ef0
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge "Add an option for the buildChooser"