Remove use of exec-root in plugins due to socket pathname limits.

Unix sockets are limited to 108 bytes. As a result, we need to be
careful in not using exec-root as the parent directory for pluginID
(which is already 64 bytes), since it can result in socket path names
longer than 108 bytes. Use /tmp instead. Before this change, setting:
- dockerd --exec-root=/go/src/github.com/do passes
- dockerd --exec-root=/go/src/github.com/doc fails
After this change, there's no failure.

Also, write a volume plugins test to verify that the plugins socket
responds.

Signed-off-by: Anusha Ragunathan <anusha@docker.com>
Upstream-commit: 21ecd5a93db34288c0c579d5738030716d7bef2d
Component: engine

components/engine/plugin/manager_linux.go

@@ -36,8 +36,6 @@ func (pm *Manager) enable(p *plugin) error {
 		return err
 	}
 
-	//TODO: check net.Dial
-
 	pm.Lock() // fixme: lock single record
 	p.P.Active = true
 	pm.save()