Fixed API bug for datasource tables queried through policy

Previously, the API would always report empty tables for
any datasource table queried through policy.  For example,
asking for all rows of the 'neutron:ports' table occurring in
the policy 'classification' would always return the empty set.

This change fixes the behavior to ensure the proper rows
are returned.  There was a typo in the get_arity function
inside the policy engine runtime: using the fully qualified
name 'neutron:ports' instead of just the table name 'ports'.

Closes-bug: 1387879
Change-Id: Id0f6b2cd095955c506d002f8f438c636ece1c9c2