LBaaS: Fix healthmonitor disassociation for non-admin

Due to specifics of policy engine, checked object should have
tenant_id to be checked by rule admin_or_owner.
In 'disassociate' operation neutron API layer works with
PoolHealthMonitorAssociation which doesn't have tenant_id field.
Need to add it to resulting dict returned by get_pool_health_monitor.

Change-Id: I6c58558b09ff34dedd7da30866275de44d3ba993
Closes-bug: 1220668
Merge "LBaaS: Fix healthmonitor disassociation for non-admin"