Use common code within manila.policy module

Rewrote 'init' and 'reset' functions for using same
functionality from common code of policy enforcer.
Changed path to test policy file, because policy enforcer
uses method of oslo.config 'find_file', that was not used before,
and does not know about test policy file.

Partially-implements blueprint use-common-code
Change-Id: I26ed170d39ed183899ee4420dc04d512cf3172e2
Merge "Use common code within manila.policy module"