Set l2 and mode info on netplugin/netmaster start time

[tiewei]

Description

Currently there are multiple l2 options in netplugin, they are vlan+bridge, vxlan+routing actually supported. With current code, when netplugin starts, it will create ovs bridge for both vlan and vxlan, and set itself as bridge mode, and when netmaster set global options, it will goto right mode and triggers netplugin re-init. This is pointless with the fact that vlan and vxlan couldn't work together at the moment, and thus bridge or routing mode has to be picked one. And you can't change the mode after network created. I think it's a lot simpler and easy to manage, when the mode are picked when netmaster/netplugin starts, and not allowed to set on the fly ( can still be reconfigured and restart serivce).

Proposal

1. make netmaster/netplugin starts with passed in mode
2. disable updating forward mode
3. creating network has to match the config netmaster being configured, ( so no need to pick vlan or vxlan to create, and will return error if request doesn't match)
4. netplugin should report its mode and netmaster will provide api to find out the ones doesn't matches its config.

[rchirakk]

I think these all boil down to one thing, not to configure default fwd-mode & netplugin waiting to init on fwd-mode config 1, 4. This can be avoided when you use the global mode 2. 👍

@gkvijay @dvavili PTAL