Fix metadata agent's auth info caching

metadata agent does not implement auth info cache correctly but
retrieves from keystone every time

Cherry-pick from https://review.openstack.org/#/c/118996/

Conflicts:
	neutron/agent/metadata/agent.py
	neutron/tests/unit/test_metadata_agent.py

Change-Id: Ifc1f580185d7600b48aaf80d112fc80e0c4253f2
Closes-bug: #1365352
Merge "Fix metadata agent's auth info caching" into stable/icehouse