snapshotter & nydusd config hot reload

[imeoer]

The nydus image is started by nydus snapshotter, and the nydusd config is currently delivered by snapshotter and is persisted to local disks, the problem is:

• the sensitive info such as registry authentication may leak on local disk.
• it is difficult to timely hot update the config if some options change dynamically.

Possible implementation steps:

• understand snapshotter's nydusd config delivery process and change it from persistent to API delivery.
• consider nydusd hot upgrade and failover process to ensure config recovery.