Refactor _lookup for secrets
Refactor _lookup for secrets

The way the _lookup function in the controllers is documented (in the
pecan documentation), the actual lookup of the entity is actually
done in that function. So this CR changes that for the "secrets"
entity. Which actually reduces code repetition throughout that
controller.

Change-Id: I4e223b106f7072ce95fd075d91dc088d90ec1b71

MCDong/barbican,cloudkeep/barbican,MCDong/barbican,openstack/barbican,cneill/barbican,cneill/barbican,cloudkeep/barbican,openstack/barbican