Agent backend abstraction

[achimnol]

Currently we use only Docker containers as the kernel session host.
However, this may be extended to other type of services, such as cloud vendor-specific remote APIs or local processes for debugging and testing purposes.
For instance, we could implement a "data agent" by specializing a subset of agents in a cluster to manage a memory-cached distributed filesystem such as Alluxio.

• Configurable driver selection on startup from agent.toml
• Generalize docker cgroup driver support #363
• Support cgroups v2 #865
• Report the list of active drivers when sending heartbeats to the manager

[achimnol]

With the work related to lablup/backend.ai-agent#66, now we have a working plugin auto-discovery mechanism.
Let's use it!

[achimnol]

This issue is the precondition to resolve lablup/backend.ai-agent#68.

[achimnol]

This is now going to be implemented via lablup/backend.ai-agent#125 and lablup/backend.ai-agent#123.

[kyujin-cho]

lablup/backend.ai-agent#123 PR now moved to lablup/backend.ai-agent#128

[achimnol]

This is considered done.
We are going to expand the format of agent.toml to have explicit backend.xxx sections for backend-specific configurations in #1597.