Enable tenants to set non-owned ext network as router gateway

Bug 1087243

Avoid loading external networks subnets in tenant's context, as this
might lead to the subnet not being found because it's owned by a
different tenant

Change-Id: Ic4c429c9e667da939ede0aa07a12c0f5a17e212c
Merge "Enable tenants to set non-owned ext network as router gateway" into stable/folsom