Refactoring execdriver.Command and Container structs to support 'docker exec' and other
similar features in the future.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Rename 'StdConfig' to 'StreamConfig'.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #7829 from crosbymichael/process-config

Refactor exec drivers for multi process