pluggable secret backend

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>
Upstream-commit: 7d45cafd5746e847e58078aa2fbdde57b5f49fa4
Component: engine

components/engine/api/types/swarm/secret.go

@@ -12,7 +12,8 @@ type Secret struct {
 // SecretSpec represents a secret specification from a secret in swarm
 type SecretSpec struct {
 	Annotations
-	Data []byte `json:",omitempty"`
+	Data   []byte  `json:",omitempty"`
+	Driver *Driver `json:"omitempty"` // name of the secrets driver used to fetch the secret's value from an external secret store
 }
 
 // SecretReferenceFileTarget is a file target in a secret reference