To keep consistent with other projects and enable 'admin_or_owner' rule
in policy.json, this patch adds neccessary fields for policy check
target and use correct method to get context dictionary.
To be clear, the rule in policy.json 'project_id:%(project_id)s' comes
from Nova originally, the policy module will get project_id from
request URL to Nova service andn compare with project_id in the user
credential. For other projects which don't contain project_id in URL,
just get it from user credential to make the rule work properly.
Change-Id: I5979f2c5204e373cc4a84b1f6997845aabc891cb
00638fa410