pkg: authorization: do not register the same plugin

This patches avoids registering (and calling) the same plugin more than
once. Using an helper map which indexes by name guarantees this and keeps
the order.
The behavior of overriding the same name in a flag is consistent with,
for instance, the `docker run -v /test -v /test` flag which register
the volume just once.
Adds integration tests.

Without this patch:
```
Dec 20 19:34:52 localhost.localdomain docker[9988]:
time="2015-12-20T19:34:52.080901676+01:00" level=debug msg="Calling
GET
/v1.22/info"
Dec 20 19:34:52 localhost.localdomain docker[9988]:
time="2015-12-20T19:34:52.081213202+01:00" level=debug msg="AuthZ
request using plugin docker-novolume-plugin"
Dec 20 19:34:52 localhost.localdomain docker[9988]:
time="2015-12-20T19:34:52.081268132+01:00" level=debug
msg="docker-novolume-plugin implements: authz"
Dec 20 19:34:52 localhost.localdomain docker[9988]:
time="2015-12-20T19:34:52.081699788+01:00" level=debug msg="AuthZ
request using plugin docker-novolume-plugin"
Dec 20 19:34:52 localhost.localdomain docker[9988]:
time="2015-12-20T19:34:52.081762507+01:00" level=debug
msg="docker-novolume-plugin implements: authz"
Dec 20 19:34:52 localhost.localdomain docker[9988]:
time="2015-12-20T19:34:52.082092480+01:00" level=debug msg="GET
/v1.22/info"
Dec 20 19:34:52 localhost.localdomain docker[9988]:
time="2015-12-20T19:34:52.628691038+01:00" level=debug msg="AuthZ
response using plugin docker-novolume-plugin"
Dec 20 19:34:52 localhost.localdomain docker[9988]:
time="2015-12-20T19:34:52.629880930+01:00" level=debug msg="AuthZ
response using plugin docker-novolume-plugin"
```

With this patch:
```
Dec 20 19:37:32 localhost.localdomain docker[16620]:
time="2015-12-20T19:37:32.376523958+01:00" level=debug msg="Calling
GET
/v1.22/info"
Dec 20 19:37:32 localhost.localdomain docker[16620]:
time="2015-12-20T19:37:32.376715483+01:00" level=debug msg="AuthZ
request using plugin docker-novolume-plugin"
Dec 20 19:37:32 localhost.localdomain docker[16620]:
time="2015-12-20T19:37:32.376771230+01:00" level=debug
msg="docker-novolume-plugin implements: authz"
Dec 20 19:37:32 localhost.localdomain docker[16620]:
time="2015-12-20T19:37:32.377698897+01:00" level=debug msg="GET
/v1.22/info"
Dec 20 19:37:32 localhost.localdomain docker[16620]:
time="2015-12-20T19:37:32.951016441+01:00" level=debug msg="AuthZ
response using plugin docker-novolume-plugin"
```

Also removes a somehow duplicate debug statement (leaving only the
second one as it's a loop of plugin's manifest):
```
Dec 20 19:52:30 localhost.localdomain docker[25767]:
time="2015-12-20T19:52:30.544090518+01:00" level=debug
msg="docker-novolume-plugin's manifest: &{[authz]}"
Dec 20 19:52:30 localhost.localdomain docker[25767]:
time="2015-12-20T19:52:30.544170677+01:00" level=debug
msg="docker-novolume-plugin implements: authz"
```

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

authorization/plugin.go

@@ -17,9 +17,14 @@ type Plugin interface {
 
 // NewPlugins constructs and initialize the authorization plugins based on plugin names
 func NewPlugins(names []string) []Plugin {
-	plugins := make([]Plugin, len(names))
+	plugins := []Plugin{}
-	for i, name := range names {
+	pluginsMap := make(map[string]struct{})
-		plugins[i] = newAuthorizationPlugin(name)
+	for _, name := range names {
+		if _, ok := pluginsMap[name]; ok {
+			continue
+		}
+		pluginsMap[name] = struct{}{}
+		plugins = append(plugins, newAuthorizationPlugin(name))
 	}
 	return plugins
 }

plugins/discovery.go

@@ -13,7 +13,7 @@ import (
 
 var (
 	// ErrNotFound plugin not found
-	ErrNotFound = errors.New("Plugin not found")
+	ErrNotFound = errors.New("plugin not found")
 	socketsPath = "/run/docker/plugins"
 	specsPaths  = []string{"/etc/docker/plugins", "/usr/lib/docker/plugins"}
 )

plugins/plugins.go

@@ -96,7 +96,6 @@ func (p *Plugin) activateWithLock() error {
 		return err
 	}
 
-	logrus.Debugf("%s's manifest: %v", p.Name, m)
 	p.Manifest = m
 
 	for _, iface := range m.Implements {