Consider the size of tmpfs when creating containers

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:

- [x] 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