Refactor plugin store to reduce nested if's in Get

This patch removes the nested if's  in the Get function
and makes the code more readable.

Signed-off-by: Boaz Shuster <ripcurld.github@gmail.com>

plugin/store.go

@@ -111,14 +111,9 @@ func (ps *Store) Remove(p *v2.Plugin) {
 
 // Get returns an enabled plugin matching the given name and capability.
 func (ps *Store) Get(name, capability string, mode int) (plugingetter.CompatPlugin, error) {
-	var (
-		p   *v2.Plugin
-		err error
-	)
-
 	// Lookup using new model.
 	if ps != nil {
-		p, err = ps.GetV2Plugin(name)
+		p, err := ps.GetV2Plugin(name)
 		if err == nil {
 			p.AddRefCount(mode)
 			if p.IsEnabled() {
@@ -133,19 +128,18 @@ func (ps *Store) Get(name, capability string, mode int) (plugingetter.CompatPlug
 		}
 	}
 
-	// Lookup using legacy model.
-	if allowV1PluginsFallback {
-		p, err := plugins.Get(name, capability)
-		if err != nil {
-			if errors.Cause(err) == plugins.ErrNotFound {
-				return nil, errNotFound(name)
-			}
-			return nil, errors.Wrap(systemError{err}, "legacy plugin")
-		}
-		return p, nil
+	if !allowV1PluginsFallback {
+		return nil, errNotFound(name)
 	}
 
-	return nil, err
+	p, err := plugins.Get(name, capability)
+	if err == nil {
+		return p, nil
+	}
+	if errors.Cause(err) == plugins.ErrNotFound {
+		return nil, errNotFound(name)
+	}
+	return nil, errors.Wrap(systemError{err}, "legacy plugin")
 }
 
 // GetAllManagedPluginsByCap returns a list of managed plugins matching the given capability.