Fix the RouterNotFound AttributeError issue

RouterNotFound is present in neutron.extensions.l3
and not in neutron.common.exceptions
Fixed this issue in get_router function
Merge branch 'router_id_error' of https://github.com/cloudwatt/contrail-neutron-plugin into router_id_error

Conflicts:
	neutron_plugin_contrail/plugins/opencontrail/ctdb/config_db.py