Consider the size of tmpfs when creating containers

[achimnol]

lablup/backend.ai-agent#94 allowed us to limit the size of /tmp directories inside containers using tmpfs (in-memory) mounts.

We need to improve the implementation:

• Make the default 64MiB size to be configurable
• Subtract the tmpfs size from the allocated memory size when creating a container
• Return an explicit error when creating containers if the allocated memory size < the tmpfs size + the image minimum memory requirement

JIRA Issue: BA-322

[achimnol]

Currently only applied when container.scratch-type is set memory, which is not default.
It is controversial to take the space for /tmp from the main memory or disk space in the container.