Refactor container preparation step

[achimnol]

The container creation step works as follows:

• XXXKernelCreationContext.spawn()
  1. Create the scratch directory for the container.
  2. Prepare /home/config/{environ,resource}.txt. (step A)
  3. Instantiate XXXKernel object to keep track of the container lifecycle.
• XXXKernelCreationContext.start_container()
  1. Finalize the port mapping.
  2. Create the container.
  3. Append the container ID to /home/config/resource.txt. (step B)
  4. Start the container.

Refactoring

• Rename spawn() to prepare_container() for clarity.
  • spawn() method does not spawn anything... It just prepares the config files in the scratch directory, and instantiate the internal record object.
• Remove container_id from KernelResourceSpec.
  • There are duplicate file-write operations for /home/config/resource.txt to just append the container ID (i.e., The step B repeats the entire step A for this). Since there are no other places that read KernelResourceSpec.container_id, we can simply remove it and just append the container ID line after creating the container without repeating the step A.