This commit extends SwarmKit secret management with pluggable secret
backends support.
Updating the work in
[swarmkit](docker/swarmkit@eebac27434) for
pluggable secret backend and adding the
driver parameter to `SecretSpec`.
Remaining work:
- [ ] CLI support (docker/cli)
- [ ] api in [plugin helpers](docker/go-plugins-helpers))
- [ ] Reference plugin
- [ ] Documenation (after cli work)
Signed-off-by: Liron Levin <liron@twistlock.com>