Change default for admin_tenant in auth_file

Currently, the tenant for the auth_file class
is set to admin by default which is inconsistent with
the default set by the keystone::roles::admin class of
openstack.

This commit updates the default value to make it more consistent.

Change-Id: I6c61794b182006d3a43212ef3e2f85c660d37831
Merge "Change default for admin_tenant in auth_file"