Initial V2 implementation of provider extension.

Initial V2 implementation of provider extension.

Initial provider extension implementation. Specify vlan_id using the
CLI with admin rights via "net-create --tenant_id <tenant-id>
<net-name> --provider:vlan_id <vlan-id>". Also includes
provider:vlan_id in reply messages for admins. The extension is
supported in the linuxbridge and openvswitch plugins.

Partially implements blueprint provider-networks.

Change-Id: I2fff64c4247b1a3091c28c7a2cd632afda192c3d