Fix default RBAC policy quota

Fix default RBAC policy quota

The previous config value for the default RBAC policy
was not in neutron.conf and value that was registered
as a config option 'rbac_entry' didn't match the resource
name 'rbac_policy' so the default did not take effect.

This patch corrects it by registering the 'rbac_policy'
option instead of 'rbac_entry' and documents it in neutron.conf.
It also adds an API test that exercises the quota limit and
ensures that it's not set to -1.

Change-Id: I8c8d4bcfda808e376af94048fe5a98c68a2a975f
Closes-Bug: #1522224