Fix owner() policy check for networks

_remove_unauthorized_networks now covers the case when
tenant_id is None.

RM9376
Quota fixes for plugin_modules/subnets.py

This is the now squashed commit of adding route_per_subnet logic
into plugin_modules/subnet.py, as well as adding new dns_nameservers
quotas. Unit tests included (RM9814 and RM9815)
RM9944

Adds connection pooling to standard and sentinel based connections
Merge pull request #249 from insequent/fix_for_500s

Fix for 500s
Merge pull request #251 from Cerberus98/rm9944_work

RM9944
Merge pull request #250 from asadoughi/9736

Fix owner() policy check for networks