vlan_transparent and mtu attributes in network_attributes

[11:00:45] <flaviof> regXboi: https://review.openstack.org/#/c/177257/ is merged. I expect issues unless 'vlan_transparent', 'mtu' is handled.

[11:01:14] <regXboi> flaviof: feel free to add the attributes to the POTS

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Send mtu and vlan_transparent for network operations

We were previously removing these. But in the spirit of transparently
passing through all attributes and API calls from Neutron to ODL,
stop pruning these.