Few changes to the UI and API implementation

1. replaced --net option for service UI with SERVICE.[NETWORK] format
2. Making using of the default network/driver backend support
3. NetworkName and NetworkType from the UI/API can be empty string
   and it will be replaced with DefaultNetwork and DefaultDriver

As per the design goals, we wanted to keep libnetwork core free of
handling defaults. Rather, the clients (docker & dnet) must handle the
defaultness of these entities.
Also, since there is no API to get these Default values from the
backend, UI will not handle the default values either. Hence, this falls
under the responsibility of the API layer to handle this specific case.

Signed-off-by: Madhu Venugopal <madhu@docker.com>

controller.go

@@ -188,6 +188,9 @@ func (c *controller) ConfigureNetworkDriver(networkType string, options map[stri
 func (c *controller) RegisterDriver(networkType string, driver driverapi.Driver, capability driverapi.Capability) error {
 	c.Lock()
 	defer c.Unlock()
+	if !config.IsValidName(networkType) {
+		return ErrInvalidName(networkType)
+	}
 	if _, ok := c.drivers[networkType]; ok {
 		return driverapi.ErrActiveRegistration(networkType)
 	}
@@ -198,7 +201,7 @@ func (c *controller) RegisterDriver(networkType string, driver driverapi.Driver,
 // NewNetwork creates a new network of the specified network type. The options
 // are network specific and modeled in a generic way.
 func (c *controller) NewNetwork(networkType, name string, options ...NetworkOption) (Network, error) {
-	if name == "" {
+	if !config.IsValidName(name) {
 		return nil, ErrInvalidName(name)
 	}
 	// Check if a network already exists with the specified network name