Add agent_id label to containers

[fregataa]

resolves #875

[achimnol]

I think we also need to consider the container's ai.backend.owner labels when scanning contaienrs in the agents.

[fregataa]

I think we also need to consider the container's ai.backend.owner labels when scanning contaienrs in the agents.

When scanning containers, should I sync kernel_registry by creating missing kernel object or just skip other's kernel object, or do both?

I let agent check container's owner id and update own container's kernel when scanning containers.