Introduce application termination message to ContainerStatus
Introduce terminationMessagePath to Container, and update conversion code
to assign the default path.

Move default setting for terminationMessagePath to conversion
from validation. Addressed other comments.
Add support to capture application termination message and propagate
the message to the upper layer through ContainerStatus.
Merge pull request #2225 from dchen1107/deathrattle

Capture application termination messages/output