Distribute with agent

[achimnol]

Historically, the kernel runner has been tightly coupled with the agent.
So updating the kernel runner happens frequently and this has imposed burdens to update & distribute new kernel images everytime even when the image's platform versions (e.g., Python version) were not changed.

So, let's install only a minimal and separate Python for the kernel runner into the kernel images and make the agent to mount the kernel runner package and its dependencies at runtime. We may need to build separate sets of Python wheels for different distros, such as Ubuntu 16.04 / 18.04 and Alpine 3.8.

• Make the agent's --debug-kernel option to be a default behavior. Add the kernel runner to the requirements of the agent.
• Change kernel images not to install the kernel runner in their Dockerfiles.
• Make a separate "kernel runner environment" Docker image which will be distributed with the agent. Its Python directories will be directly mounted into kernels at runtime. Update the agent to work with it.
  • Could we make the pip install process to automatically pull or build the kernel runner environment Docker image?
• Update the offline installers and the development setup script in the meta-repository.
• Update the documentation.

[achimnol]

Backend.AI-related, agent-mounted contents will be placed at /opt/backend.ai in kernel containers to avoid conflicts with existing softwares in the kernel images.

[achimnol]

This work will be first tested with NGC images (#15).