Disabled volume discovery and added env var to enable it if needed

Volume discovery was written to address challenges with docker/plugin startup order,
and as potential auto-recovery for plugin crashes.

* We do not see crashes often (as a matter of fact, only once so far , with VMCI memory issues  - and
it' fixed).
* Docker now manages the order of plugin invocation/startup with
Managed Plugins.
* and, the last but not the least, during the discovery we make assumption about Docker API availability,
but Docker makes API available only AFTER full startup including plugin initialization, so it could lead to
bug like #1039, where docker either hangs for 15-20 sec (like with legacy plugin) or steps on it's own
bugs and crash (like with managed plugin)

In the spirit of taking care of the "sunny day" first, when everything works OK and our code does not crash,
this commit turns off discovery on startup.

Managed plugin handles all restarts and remounts fine. Legacy plugin may have issues if we crash, but first of all
we never crash :-) and second legacy plugins are getting depreciated anyways

plugin/config.json

@@ -53,7 +53,11 @@
       "description": "Log level - info, warn, error or debug",
       "value": "info",
       "Settable": [ "value"]
-	}
-	]
+	},
+	{"name": "VDVS_DISCOVER_VOLUMES",
+      "description": "non-empty if pre-existing volumes need to be discovered ",
+	  "value": "",
+      "Settable": [ "value"]
+	}]
 }
 

vmdk_plugin/utils/refcount/refcnt.go

@@ -74,6 +74,7 @@ package refcount
 import (
 	"fmt"
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"strings"
 	"sync"
@@ -152,6 +153,11 @@ func newRefCount() *refCount {
 // This functions does not sync with mount/unmount handlers and should be called
 // and completed BEFORE we start accepting Mount/unmount requests.
 func (r RefCountsMap) Init(d drivers.VolumeDriver, mountDir string, name string) {
+	e := os.Getenv("VDVS_DISCOVER_VOLUMES")
+	if  e == "" {
+		log.Debug("RefCountsMap.Init: Skipping Docker volumes discovery - VDVS_DISCOVER_VOLUMES not set")
+		return
+	}
 	c, err := client.NewClient(DockerUSocket, ApiVersion, nil, defaultHeaders)
 	if err != nil {
 		log.Panicf("Failed to create client for Docker at %s.( %v)",