This fix tries to fix the issue raised in 28684:
1. Duplicate plugin create with the same name will override the old plugin reference
2. In case an error happens in the middle of the plugin creation, plugin directories
in `/var/lib/docker/plugins` are not cleaned up.
This fix update the plugin store so that `Add()` will return an error if a plugin
with the same name already exist.
This fix also will clean up the directory in `/var/lib/docker/plugins` in case
an error happens in the middle of the plugin creation.
This fix fixes 28684.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
