Allow reading a tenant router's external IP

Adds an external IPs field to the external gateway information
for a router so the external IP address of the router can be
read by the tenant.

DocImpact

Closes-Bug: #1255142
Change-Id: If4e77c445e9b855ff77deea6c8df4a0b3cf249d4
Fix setup of Neutron core plugin in VPNaaS UT

One of the VPNaaS unit test setup routines creates
and extension manager but passes it a class but it
should be passing an instance of that class.

Change-Id: I589cdda0674fdf8fa20d92c2609e1ba6966125d8
Merge "Fix setup of Neutron core plugin in VPNaaS UT"
Merge "Allow reading a tenant router's external IP"