Imports oslo policy to fix test issues

Tests inheriting from keystone.tests.TestCase depend on having the
policy_file command-line option declared before setUp runs. Importing
the oslo policy module automatically declares the option.

Fixes-Bug: 1233224
Change-Id: I939bbdcb7df1c4a59108dced92d4d5c2e96defe2
Merge "Imports oslo policy to fix test issues"