The Tasks API was made admin-only in Mitaka to prevent it from being
exposed directly to end users. The interoperable image import
process introduced in Pike uses the tasks engine to perform the
import. This patch introduces a new policy, 'tasks_api_access',
that determines whether a user can make Tasks API calls.
The currently existing task-related policies are retained so that
operators can have fine-grained control over tasks. With this
new policy, operators can restrict Tasks API access to admins,
while at the same time, admin-level credentials are not required
for glance to perform task-related functions on behalf of users.
Change-Id: I3f66f7efa7c377d999a88457fc6492701a894f34
Closes-bug: #1711468
b90ad2524f