Enable resource usage tracking for reference plugins.

Enable resource usage tracking for reference plugins.

Specify which resources should be tracked when initializing
the ML2 and l3_router plugins. This will enable usage tracking
for the following resources:
- Networks
- Ports
- Subnets
- Subnet pools
- Security groups
- Security group rules
- Routers
- Floating IPs

Partially implements blueprint: bp/better-quotas

Change-Id: I57287b15ffdadc30297651a01e9f05110e9ce6b6