Remove long db transaction for metadata access network

Bug 1204277

Removes nested transactions wrapping plugin ops, and adds
rollback code where required.
Also ensures NeutronPlugin.py does not attempt to remove router
ports twice.

Change-Id: I299d4ed688a70b6dff506c999355661cf783ae26
Do not call remove_router_lport in remove_router_interface

Bug 1211149

The call will cause a failure as the router port is already removed
by _nvp_delete_router_port.
This patch also add the support on the fake nvp api client to verify
this kind of condition.

Change-Id: Ieb5ca5c239ccc993b96dd5bad0b412211b45417e
Merge "Do not call remove_router_lport in remove_router_interface"
Merge "Remove long db transaction for metadata access network"