Add enable_template option to the config file.
When it is set, enable the vm template factory.
cache factory cannot be used by kata cli directly because
it requires a running daemon to maintain the cache VMs.
`kata-runtime factory init` would initialize the vm factory and
`kata-runtime factory destroy` would destroy the vm factory.
When configured, a vm factory is loaded before creating new sandboxes.
Signed-off-by: Peng Tao <bergwolf@gmail.com>
