sqlite: add a hidden --db-backend flag

Add a hidden flag to set the database backend and plumb it into
podman-info.  Further add a system test to make sure the flag and the
info output are working properly.

Note that the test may need to be changed once we settled on how
to test the sqlite backend in CI.

Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>

pkg/domain/entities/engine.go

@@ -34,6 +34,7 @@ type PodmanConfig struct {
 
 	ContainersConf           *config.Config
 	ContainersConfDefaultsRO *config.Config // The read-only! defaults from containers.conf.
+	DBBackend                string         // Hidden: change the database backend
 	DockerConfig             string         // Used for Docker compatibility
 	CgroupUsage              string         // rootless code determines Usage message
 	ConmonPath               string         // --conmon flag will set Engine.ConmonPath