Migrate login & logout command to cobra

Also moves some common stuff around :
- `api/client/registry.go` for registry related
method (`ElectAuthServer`, …)
- `api/client/credentials.go` to interact with credentials

Migrate logout command to cobra

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

usage.go

@@ -13,8 +13,6 @@ var DockerCommandUsage = []Command{
 	{"exec", "Run a command in a running container"},
 	{"info", "Display system-wide information"},
 	{"inspect", "Return low-level information on a container or image"},
-	{"login", "Log in to a Docker registry"},
-	{"logout", "Log out from a Docker registry"},
 	{"ps", "List containers"},
 	{"pull", "Pull an image or a repository from a registry"},
 	{"push", "Push an image or a repository to a registry"},