Fix implicit DeviceMapper selection

DeviceMapper must be explicitly selected because the Docker binary might not be linked to the right devmapper library.

With this change, Docker fails fast if the driver detection finds the devicemapper directory but the driver is not the default option.
The option `override_udev_sync_check` doesn't make sense anymore, since the user must be explicit to select devicemapper, so it's being removed.
Docker fails to use devicemapper only if Docker has been built statically unless the option was explicit.

Signed-off-by: David Calavera <david.calavera@gmail.com>
Upstream-commit: 0a376291b2213699f986a7bca1cc8c4f4ed00f8d
Component: engine

components/engine/daemon/graphdriver/devmapper/devmapper_test.go

@@ -13,7 +13,6 @@ func init() {
 	DefaultDataLoopbackSize = 300 * 1024 * 1024
 	DefaultMetaDataLoopbackSize = 200 * 1024 * 1024
 	DefaultBaseFsSize = 300 * 1024 * 1024
-	DefaultUdevSyncOverride = true
 	if err := graphtest.InitLoopbacks(); err != nil {
 		panic(err)
 	}