Add LeaveAll support

Currently container can join one endpoint when it is started.
More endpoints can be attached at a later point in time. But
when that happens this attachment should only have meaning
only as long as the container is alive. The attachment should
lose it's meaning when the container goes away. Cuurently there
is no way for the container management code to tell libnetwork
to detach the container from all attached endpoints. This PR
provides an additional API `LeaveAll` which adds this
functionality,

To facilitate this and make the sanbox lifecycle consistent
some slight changes have been made to the behavior of sandbox
management code. The sandbox is no longer destroyed when the
last endpoint is detached from the container. Instead the sandbox
ie kept alive and can only be destroyed with a `LeaveAll` call.
This gives better control of sandbox lifecycle by the container
management code and the sandbox doesn't get destroyed from under
the carpet while the container is still using it.

Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>

error.go

@@ -51,7 +51,7 @@ func (ij ErrInvalidJoin) BadRequest() {}
 type ErrNoContainer struct{}
 
 func (nc ErrNoContainer) Error() string {
-	return "a container has already joined the endpoint"
+	return "no container is attached to the endpoint"
 }
 
 // Maskable denotes the type of this error