Add import for filesystem_store_datadir config.

Adds a missing CONF import for the filesystem_store_datadir.
This is required in order to make the policy tests (test_policy.py)
run as a stand alone suite.

Fixes: NoSuchOptError: no such option: filesystem_store_datadir

Fixes LP Bug #1098670.

Change-Id: If8f9db3962b070e7841e4e3ef531dda4645624b7
Merge "Add import for filesystem_store_datadir config."