Local Path Provisioner v0.0.18 release

Changelog:
1. Thanks to @nickming 's PR, we've added the ability to make the helper
pod and setup/teardown script customizable. For example, now you can
setup the local path provisioner to use quota with btrfs.

2. Thanks to @hwaastad 's PR, now the helper pod supports pull secret.

Signed-off-by: Sheng Yang <sheng.yang@rancher.com>