This fix is part of the discussion in 28199 about using
`truncate` to replace `--no-trunc`.
As part of the fix, a new function `truncate` has been
added for Go templates so that it is possible to use
```
docker stack services --format "{{truncate .ID 5}}: {{.Mode}} {{.Replicas}}"
```
to show truncated ID.
A unit test has been added.
This fix is related to 28199.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Upstream-commit: 7fa8d5e064bad6d65348a69b623ebb2ecffdf248
Component: engine
