This fix tries to address the issue raised in 28581 and 28927
where it is not possible to create a secret from a file (only
through STDIN).
This fix add a flag `--file` to `docker secret create` so that
it is possible to create a secret from a file with:
```
docker secret create --file secret.in secret.name
```
or
```
echo TEST | docker secret create --file - secret.name
```
Related docs has been updated.
An integration test has been added to cover the changes.
This fix fixes 28581.
This fix is related to 28927.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
