Add capability to specify mount propagation per volume

Allow passing mount propagation option shared, slave, or private as volume
property.

For example.
docker run -ti -v /root/mnt-source:/root/mnt-dest:slave fedora bash

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

types/types.go

@@ -338,6 +338,7 @@ type MountPoint struct {
 	Driver      string `json:",omitempty"`
 	Mode        string
 	RW          bool
+	Propagation string
 }
 
 // Volume represents the configuration of a volume for the remote API