Add sample admin_{user,tenant_name,password}

Other OpenStack components have the same configs in api-paste.ini.
admin_token was removed by Bug #1299332, and admin_{user,password,
tenant_name} should be added instead of admin_token.

Change-Id: Icabb056c315556db8064ded882fba13c25ec2bca
Closes-Bug: #1325482
Merge "Add sample admin_{user,tenant_name,password}"